Disc brake conversion is driving me crazy!

[HSRACER]

So I bought a disc brake conversion kit and installed the rotors while leaving the manual master cylinder on as I ran out of time the day I did the conversion.

Last week I finished adding the booster bracket, booster and the new master cylinder.

The power brakes are giving me fits. After I put discs on it and left the manual drum/drum master cylinder on it, the truck stopped great and straight but took a lot of pedal pressure.

Today I bled the new system a couple of times and there is absolutely no air in the system.

Now it stops ok but takes crazy amount of pedal pressure. Worse than when the booster wasn’t installed. Acts like the booster isn’t working at all. I have a ton of vacuum at the booster.

It still has the drum/drum proportioning valve on it. I had that on it when I did the disc conversion only and it stopped great. I don’t understand what could have gone wrong.

Any help is appreciated.

[HSRACER]

I mentioned the proportioning valve because I got a ton of feed back saying it is fine to leave on, or take it completely off, or do a disc/drum one. As I mentioned above it worked well before I added the new power booster and the seller of the kit told me the valve is basically a pass through on drum drum so it is fine to leave it installed.

I can’t really drive the truck since I added the booster as it takes so much pedal pressure now that I don’t feel safe in it!

[67ChevyRedneck]

You need a factory style disc brake proportioning valve:

A Proportioning valve is required on vehicles that have disc brakes on the front wheels and drum brakes on the rear wheels. Disc brake pads are normally in contact with the disc, while the drum brake shoes are normally not in contact with the drum. If the pressure was not proportioned the disc brakes would engage before the drum brakes when you depress the brake pedal.
The Proportioning valve compensates for this, allowing the drum brakes to engage first before the disc brakes. The Proportioning valve does not allow any pressure to the disc brakes until a pre-determined pressure has been reached. The pre-determined pressure is low when compared to the maximum pressure in the braking system, this allows the drum brakes to engage before the disc brakes engage. Having the rear brakes engage first provides the control and stability needed to stop your vehicle safety.
The proportioning valve reduces the pressure to the rear brakes. Whichever type of brakes your vehicle has, the rear brakes require less pressure than the front brakes.
If equal braking force were applied to all four wheels during a stop, the rear wheels would lock-up before the front wheels. The proportioning valve only lets a portion of the amount of pressure to the rear wheels thus preventing rear wheel lock-up.

http://www.classicperform.com/How/Ho...-Prop-Work.htm

[lolife99]

That’s a distribution block from a drum/drum truck.
You can keep it and add an adjustable prop valve or switch to a disc/drum prop valve.
Should make it brake better,... but not sure why you have such a hard pedal.

[kazoocruiser]

You had a drum/ drum setup, which means all shoes extended at the same time. Because you now have disk / disk, all pads are in contact with the disks. Seems that lines up with what you indicated "After I put discs on it and left the manual drum/drum master cylinder on it, the truck stopped great and straight but took a lot of pedal pressure.

Now it stops ok but takes crazy amount of pedal pressure. Worse than when the booster wasn’t installed. Acts like the booster isn’t working at all. I have a ton of vacuum at the booster.

It still has the drum/drum proportioning valve on it. I had that on it when I did the disc conversion only and it stopped great."

------

How do you define a lot of vacuum at the booster? Where did you port your source from?

Maybe the booster needs a boost.

How far does the pedal drop when you push it? Is it consistent in distance with what it was before you changed out the master cylinders?

Have you tried to pump the pedal while stopping to multiply the pressure in the lines?

Is it possible you have carpet or floormats bunched up on the floor under the pedal? That happens too.

[kazoocruiser]

It occurred to me to add this. With the engine running and parked, pump the brakes and hold the pedal, then shut off the engine, and wait to see how long it takes for the pedal to move. This might isolate a vacuum leak internally to the booster.

Did you install a check-valve in the booster line? Is it installed backwards?

What all came in this kit, and where did you get it?

It wouldn't take much of a pinhole in the booster diaphragm to cause a loss of vacuum assist.

[RichardJ]

Did the new master have the shallow hole needed for a P-booster? The pin on the booster needs to be checked for proper length.


Assuming the master is correct and the booster pin is checked for correct length;

Boosters are designed to hold enough vacuum for three applications of the brake pedal if the engine should die.
With engine OFF, apply the brakes 3-4 times and you should have a high firm pedal.
While holding the brake pedal down, start the engine. Pedal should drop as engine vacuum builds.

[HSRACER]

Thank you all for the current feedback.

I understand what a proportioning valve does and how there is a residual pressure difference to keep,the rear drums engaging first to control brake application and potential lock up of the wheels in a panic situation.

The difference in distribution blocks would have nothing to do with the amount of pressure it takes to slow the vehicle.

In all my other trucks with the same disc drum combination, I can barely move the pedal with almost no effort and they slow as if driving a new vehicle.

The baffling thing on this setup is it actually got worse and harder after adding the new master and booster.

The new master has the correct shallow piston in it and the pivot bracket you can see in the pictures is designed to duplicate the correct ratio of travel just like the factory bracket.

I made sure the push rod end was not touching the master cylinder plunger at installation.

Also I pulled the vacuum off of the factory rear port of the LS intake just like it is from the factory.

The push rod was out too far from the original install but only about a 16th of an inch. There was an adjustment nut at the end of the pushrod and I backed that in as far as it would go so it did not cause a drag or slight engagement when at rest.

The next steps sound like I need to start the engine and check the booster after shut down as well as hold the pedal down and see if the travel changes at start up.

This thing feels as if the booster isn’t eve there. If you have ever had you vehicle die while going down the road and had to use the brakes with no vacuum it is very hard to stop. That’s how this feels all the time.

I did have to use a 90 degree booster valve coming off the back of the intake to get the hose hooked up for firewall clearance. I removed the flapper rubber out of it so it is now basically just an open 90 degree connector as I knew it wouldn’t work with the rubber flapper valve in there.

The booster still has the correct flapper valve on it.

[lilredfifty]

Sometimes even a new booster can be bad. Listen for any hissing sounds at the booster. You can hook a vacuum pump to the booster and pull vacuum and see if it holds.

[HSRACER]

Thought that I would add this info as well.

I bleed the mater cylinder with a bleed kit that routed the fluid back into the the reservoir until there was no air in it at all.

Then did a manual gravity drain at each line connection of the master cylinder and all of the proportioning valve.

Next we started at the passenger rear brake and bleed it until no sign of air was seen. We then did each wheel until all fluid was clear and free of air.

In my mind everything is pointing to a bad booster or a push rod travel issue but I am unsure of how to know for sure.

[Sspirawk]

In my past experience. If the pedal does not drop at start up you have no vacuum, a stock ls shoud generate more then enough vacuum in just a few seconds. Dont think a secondary booster is needed. My money is in the booster. To check I would rig up a brake line vac tool with a gauge. And try to get 20 25 in/hg if u cant prolly bad diaphragm. If you can. And it holds. And you can pump the MC by hand. Then I would hazard a guess on push rod binding or no travel. Either way it points to the booster.

[68bbc/20]

did you install disc brakes on all 4 or just the front? drums and disc combo
require different amounts of fluid to each set to work correctly. that why they have a prop/ valve.

[AussieinNC]

I just reread your post about removing the rubber flapper valve from the booster connector....sad to say, but that is most likely the issue...

If you told the sales person the engine was an LS and you needed a booster for that engine, that valve MUST be in place....I can almost guarantee the booster connection is just a 90 degree fitting, with the booster check valve expected to be in the engine port....try it and see...

I will leave the rest of the original response for everyone general knowledge...

The standard brake pipe block on a drum/drum truck is not a proportioning valve....it is a simple trip over alert system to warn of a brake system pressure loss in either the front or rear brake circuits.

Often when bleeding a system the trip valve will fail over and block off one of the brake circuits....to correct this, remove the electrical switch from the block then pump the brake pedal several times.

A disc/ drum system must have a proportioning valve in the system to prevent excessive pressure generated by the brake system from locking up the rear brakes. The proportioning valve usually has a brake fail switch included...and again, often these drop into fail mode when someone is bleeding the system.

I always remove the switches before bleeding any system, bleed then once fully bled, replace the switch. I also prefer a pressure bleeder system over pumping pedal style of process.

A disc/drum master cylinder is a totally different item to a drum/drum master.
One of the most often overlooked items to check is the booster push rod depth into the master cylinder. It must be correct...too little depth into the cylinder and you will lose cylinder travel....too much depth into the master and the front disc system may not return correctly and lock up the front system.

Another often overlooked item is the pedal pivot position to the pushrod that goes into the cab side of the booster...an incorrect position of the pivot point will result in hard pedal or loss of travel....

Did you check the amount of protusion of the booster to master cylinder push rod?
Show us pics of both the back of the master cylinder and the front of the booster please.

Show us where the vacuum hose from the engine connects to the engine and then to the booster...close up pics please...

Have you taken a vacuum reading of the available vacuum from the engine at idle, from the booster hose fitting on the engine and again at the end of the hose where it meets to booster...

Thanks

[HSRACER]

Originally Posted by AussieinNC

I just reread your post about removing the rubber flapper valve from the booster connector....sad to say, but that is most likely the issue...

If you told the sales person the engine was an LS and you needed a booster for that engine, that valve MUST be in place....I can almost guarantee the booster connection is just a 90 degree fitting, with the booster check valve expected to be in the engine port....try it and see...

I will leave the rest of the original response for everyone general knowledge...

The standard brake pipe block on a drum/drum truck is not a proportioning valve....it is a simple trip over alert system to warn of a brake system pressure loss in either the front or rear brake circuits.

Often when bleeding a system the trip valve will fail over and block off one of the brake circuits....to correct this, remove the electrical switch from the block then pump the brake pedal several times.

A disc/ drum system must have a proportioning valve in the system to prevent excessive pressure generated by the brake system from locking up the rear brakes. The proportioning valve usually has a brake fail switch included...and again, often these drop into fail mode when someone is bleeding the system.

I always remove the switches before bleeding any system, bleed then once fully bled, replace the switch. I also prefer a pressure bleeder system over pumping pedal style of process.

A disc/drum master cylinder is a totally different item to a drum/drum master.
One of the most often overlooked items to check is the booster push rod depth into the master cylinder. It must be correct...too little depth into the cylinder and you will lose cylinder travel....too much depth into the master and the front disc system may not return correctly and lock up the front system.

Another often overlooked item is the pedal pivot position to the pushrod that goes into the cab side of the booster...an incorrect position of the pivot point will result in hard pedal or loss of travel....

Did you check the amount of protusion of the booster to master cylinder push rod?
Show us pics of both the back of the master cylinder and the front of the booster please.

Show us where the vacuum hose from the engine connects to the engine and then to the booster...close up pics please...

Have you taken a vacuum reading of the available vacuum from the engine at idle, from the booster hose fitting on the engine and again at the end of the hose where it meets to booster...

Thanks

So to answer all your questions they are as follows

The rod into the master from the booster was what I mentioned earlier that I adjusted with the small nut on the end of the booster coming out the front of the booster. There was not enough clearance for me to bolt up the master cylinder flush to the booster so I backed of the small nut and screwed in the shaft to add clearance to the master. Now it bolts flush and has no preload like before.

The vacuum port was plugged on the engine and has a large hose port. I needed a large hose to medium hose connector there so the 90 was perfect for getting that accomplished. I Knew the flapper had to be removed to,actually get vacuum to the booster. The booster came with a ported 90 when I got the kit so it has the flapper still in it. Basically it is hooked up as a normal booster line with the flapper in the booster 90.

The vacuum gauge shows 19 inch pounds at the intake and at the booster valve.

The vacuum line can be disconnected from the intake and it will hold vacuum in the booster as it should. This indicates the flapper is working as it should on the booster.

The booster holds vacuum long after the engine is turned off. I went out a few minutes after shut down and loosened the 90 at the booster slowly and heard the vacuum leak out slowly as I removed the vacuum connection.

The pedal adjustment inside the cab was adjusted to move the pedal in towards the firewall an inch or two to give me more leg room. The shaft to the outside pivot was adjusted to be sure it was neutral inside the cab and not putting any preload on the outside shaft going towards the booster.

I have not tried to remove the switch at the distribution block yet and I will try that in a few hours. The bleed flow on all the wheels was strong when we bled them which I figured would eliminate any issue with the distribution block?

So for I have either verified or done what everyone has mentioned.

Is there a way of knowing what happens when a booster is bad. Normally in my experience they leak or hiss and offer no assistance at the pedal.

This one offers no support at the pedal and the brake feel is the same with the vacuum hooked up and unhooked. That keeps pointing to the booster in my mind.

I will keep taking suggestions until we figure this out��

[HSRACER]

Originally Posted by 68bbc/20

did you install disc brakes on all 4 or just the front? drums and disc combo
require different amounts of fluid to each set to work correctly. that why they have a prop/ valve.

This is a disc drum set up now. When stopping hard the tears will lock up so I will be adding a proportioning valve in the future.

The portioning valve should have nothing to,do,with a hard pedal pressure issue though.

[kazoocruiser]

Back from some research . . .

From what little i can see, you have isolated the problem to the lack of vacuum assist. What I was able to find in looking at what I could find regarding the booster itself, is that under normal circumstances, the diaphragm is considered "suspended", and receives vacuum on either side. That changes when the brakes are applied. There is a port to atmospheric pressure that is supposed to open, breaking the vacuum to the back of the diaphragm, allowing a pressure differential into the unit, collapsing the vacuum on the driver's side of the diaphragm. In your case, apparently that isn't happening.

It could be the internal valve is defective, or paint on the booster itself has blocked a pinhole that is supposed to be open to the outside. I don't know if that is the case, but it might be worth pulling off the booster itself and doing an inspection of the firewall side of the unit to see if there is a staple-sized hole that is covered with paint, which has sealed the unit and is preventing it from venting.

The hole won't normally admit air, but if it is sealed, it won't work when it is supposed to.

[RichardJ]

>>A disc/drum master cylinder is a totally different item to a drum/drum master.<<

That, IS NOT true. A disc/drum MC may or it may not have a rear reservoir that is larger than the front reservoir. The size of the reservoir has absolutely nothing to do with the operation of the brake system. 30,000 miles down the road, when the disc pads have worn down to nothing, a reservoir may run out of fluid if it is too small. Up until the time the reservoir runs out of fluid and starts pumping air, the system will continue functioning normally.

Disc brakes do require a little more pressure. If a P-booster isn't used, pressure can be increased by using a MC with slightly smaller piston bore diameter. The down-side of a smaller piston bore is that it requires greater foot/pedal travel.
If a P-booster is added, there is a slight travel distance loss through the booster. Adding a booster generally calls for larger MC piston bore diameter. The larger piston bore requires greater foot/pedal pressure, but with less pedal travel distance.

There is a difference between MC's used with a booster or with manual. CPP corrects that difference by fitting a deep piston bore manual MC with a plug that the allows it to be used with a booster.

You guys may want to rethink your adjustment of the booster-to-MC pi adjustment.
It needs to be measured or at lease be compared with a precise measurement. There are tools made for this or you can use standard measurement instruments.
I don't have the tool. I use a slight variation on the method shown to measure the depth of the piston hole.

[Chris_oz]

It sounds like you have checked almost everything.

with the engine off, pump the pedal several times and release, then do it again, until you have a nice high firm pedal and the vacuum is gone from the booster.

Keeping your foot firm on the pedal, start the engine.

The pedal must sink down towards the floor, this indicates that the booster is Boosting.
If the pedal stays at the same height, booster is not working.

Double check the hose to the booster, make sure it is vacuum hose, non vacuum hose will collapse and block vacuum, sometimes a small amount can get through, this is enough to trick you to think its all good.

[HSRACER]

I fixed the brakes today!

I got to thinking about what else I had changed or modified when doing the install and I remembered that the booster did not have a rubber cover on the back of it like the drum/drum master had. It left a large gap between the rod and the firewall hole which would let tons of hot air into the cab.

I had found a rubber cup that fit perfectly around the shaft and the booster end.

That was the problem area. The cup was not allowing any air to enter the backside of the booster which effectively eliminated the booster all together.

The booster was getting plenty of vacuum but no air on the backside. I cut off the cup and bam I had super strong light effort brakes as designed.

So the whole problem was my fault for adding the cup when it did not need one.

Thank you all for your assistance!

[HSRACER]

Originally Posted by kazoocruiser

Back from some research . . .

From what little i can see, you have isolated the problem to the lack of vacuum assist. What I was able to find in looking at what I could find regarding the booster itself, is that under normal circumstances, the diaphragm is considered "suspended", and receives vacuum on either side. That changes when the brakes are applied. There is a port to atmospheric pressure that is supposed to open, breaking the vacuum to the back of the diaphragm, allowing a pressure differential into the unit, collapsing the vacuum on the driver's side of the diaphragm. In your case, apparently that isn't happening.

It could be the internal valve is defective, or paint on the booster itself has blocked a pinhole that is supposed to be open to the outside. I don't know if that is the case, but it might be worth pulling off the booster itself and doing an inspection of the firewall side of the unit to see if there is a staple-sized hole that is covered with paint, which has sealed the unit and is preventing it from venting.

The hole won't normally admit air, but if it is sealed, it won't work when it is supposed to.

You were right on track with your suggestions. I figured it out earlier today and just now saw your post!

See my post right before this one for details.

Thank you.

[kazoocruiser]

Originally Posted by HSRACER

You were right on track with your suggestions. I figured it out earlier today and just now saw your post!

See my post right before this one for details.

Thank you.

I think you can thank Captain Canabis for His helpful
contribution.

[Billyburban]

Originally Posted by HSRACER

I fixed the brakes today!

I got to thinking about what else I had changed or modified when doing the install and I remembered that the booster did not have a rubber cover on the back of it like the drum/drum master had. It left a large gap between the rod and the firewall hole which would let tons of hot air into the cab.

I had found a rubber cup that fit perfectly around the shaft and the booster end.

That was the problem area. The cup was not allowing any air to enter the backside of the booster which effectively eliminated the booster all together.

The booster was getting plenty of vacuum but no air on the backside. I cut off the cup and bam I had super strong light effort brakes as designed.

So the whole problem was my fault for adding the cup when it did not need one.

Thank you all for your assistance!

Good morning I’m having this same issue I swapped out all to 4wheel disc cpp prop valve, the disc disc matercylinder an a c20/c30 booster an there’s brakes but not would I expected from 4 wheel disc set up. What booster did you use ? Can you post pics of what fixed your problem please 🙏🏻