Another brake question

[Ghost's68]

Hey guys I got a question about my brakes. I posted earlier asking for help ID'ing some rear components. I was able to replace all the parts but here's is where my question comes into play.

So after power bleeding my brake system using a motive bleeder, I can't get the brake pedal to stiffen up. It just keeps going to the floor and when I try to stop the vehicle I put my foot damn near the floor with no lock up what so ever.

My dual M/C has a separate lines that runs to the front and the other to the rear. The lines from M/C do not go to Prop valve of any type. They go straight to the caliper. Am I missing something?

[Stocker]

What year truck? Up through 1970 was drum/drum and did not use a prop. valve (but may have a distribution block). Front discs started in 1971 and used a prop. valve for increased line pressure to the calipers.

Regardless, pedal shouldn't be going to the floor. Sounds like maybe still air in the lines somewhere. Did you replace -- and bench bleed -- the master cylinder?

[Ghost's68]

Truck is a 68 with 4 wheel disc brakes

[compsystems]

It sounds like you have air trapped in the top of the calipers. Make sure the bleed screw is at the absolute highest point to release the trapped air.

If its not air, then its a bad master cylinder.

[Ghost's68]

Originally Posted by compsystems

It sounds like you have air trapped in the top of the calipers. Make sure the bleed screw is at the absolute highest point to release the trapped air.

If its not air, then its a bad master cylinder.

All 4 corners have the bleeder pointing up. After first bleeding the system I went for a test drive and I re-bled the system again. Still same problem

[LongBox]

I assume there are no brake fluid leaks, or you'd have mentioned that. Does pumping the pedal firm up the feel? If so, that is a definite sign of air in the system. If that doesn't work, you most likely have a bad MC. Time to swap it out, and go through the whole bleeding process again, starting with bench bleeding the MC.

[weq92f]

Definitely try the pump-up method to see if you can stiffen the pedal with just pumping it. If you can, that would point to a bad MC. I drove a manual brake 68 c10 using the pump-it-up method for years. Replaced the MC and never had to pump it again!

Maybe try bleeding the brakes the old fashion way: use helper at brake pedal.

Hth,

-weq

[Ghost's68]

I try and just pump up the brakes while the truck is on and every time it goes to the floor. I've been reading up and I keep seeing that a prop valve is needed for disc/disc setup.

[RichardJ]

>>used a prop. valve for increased line pressure to the calipers.<<

A proportioning valve is not connected to the front brakes and does not affect the front brakes in any way. A proportioning valve reduces pressure to the rear brakes, BUT only above a certain pressure. A typical fixed proportioning valve will begin to operate at around 500#. IE when you have 400#on the front brakes, you will have 400# on the rear brakes. When you have 600# on the front brakes, the proportioning valve will reduce the pressure to the rears to maybe 575#.

If your problem was that the rear brakes were locking up, under moderate to heavy braking, then you would need a proportioning valve. At this point, you are reporting no need for a proportioning valve.

An aftermarket adjustable proportioning valve gives you the ability to shift the starting pressure point up or down as needed.

Does it have a power booster?

As this is not a factory setup, start at the pedal. Is the free-play adjusted properly? Is there slop in the pedal linkage? If there is a booster, is the clearance between the booster pin and the MC adjusted properly.

As the others have mentioned, have you checked to see if the pressure can be pumped up?

[zosoppp]

For the record my 1970 with power drum/drum also has some sort of valve between the MC and wheel cylinders.

It's also where the brake warning light sensor/switch is mounted.

[joeminnesota]

Hello, and i hope you get this sorted out.
If you are sure there are no VISIBLE leaks and there is NO air in the system, that leads me to believe the Master CYL is bad. It could be leaking (bypassing) internally or leaking between the Master Cyl and the Booster. How old is the Master Cyl? If it is new, did you bench bleed it?

[Ghost's68]

Originally Posted by RichardJ

>>used a prop. valve for increased line pressure to the calipers.<<

A proportioning valve is not connected to the front brakes and does not affect the front brakes in any way. A proportioning valve reduces pressure to the rear brakes, BUT only above a certain pressure. A typical fixed proportioning valve will begin to operate at around 500#. IE when you have 400#on the front brakes, you will have 400# on the rear brakes. When you have 600# on the front brakes, the proportioning valve will reduce the pressure to the rears to maybe 575#.

If your problem was that the rear brakes were locking up, under moderate to heavy braking, then you would need a proportioning valve. At this point, you are reporting no need for a proportioning valve.

An aftermarket adjustable proportioning valve gives you the ability to shift the starting pressure point up or down as needed.

Does it have a power booster?

As this is not a factory setup, start at the pedal. Is the free-play adjusted properly? Is there slop in the pedal linkage? If there is a booster, is the clearance between the booster pin and the MC adjusted properly.

As the others have mentioned, have you checked to see if the pressure can be pumped up?

Thanks for the simple breakdown on prop valve and if needed. Since the first day I purchased the truck the brakes have seemed off and very light. So for now I'll focus on the breaks being light and not locking up.

Originally Posted by zosoppp

For the record my 1970 with power drum/drum also has some sort of valve between the MC and wheel cylinders.

It's also where the brake warning light sensor/switch is mounted.

Originally Posted by joeminnesota

Hello, and i hope you get this sorted out.
If you are sure there are no VISIBLE leaks and there is NO air in the system, that leads me to believe the Master CYL is bad. It could be leaking (bypassing) internally or leaking between the Master Cyl and the Booster. How old is the Master Cyl? If it is new, did you bench bleed it?

I hope so too since not having the full functionality of brakes can be a very dangerous thing. I wouldn't want to find out what can happen. I have bleed the system twice using the motive power bleeder and both times let the pump sit at about 15psi checking for some type of leak. Didn't find any. The truck was purchased and not sure what the PO did when he did the swap. I'm just going through and cleaning up all the systems slowly. It does have a booster. M/C does not look new to me. But maybe it was when purchased. I'll try and snap a pic of what I'm looking at here for you guys.

[Stocker]

Originally Posted by zosoppp

For the record my 1970 with power drum/drum also has some sort of valve between the MC and wheel cylinders.

It's also where the brake warning light sensor/switch is mounted.

That is a distribution block.

[joeminnesota]

What components did you replace?

It is reasonable to suspect the last replaced component if the new symptom occurred after that installation.

If none of the new components are faulty, i would replace the Master Cyl.

Air in the system is indicated by a soft pedal that when pumped a few times becomes firm, after all, air is compressible. A leak, weather internal to the Master Cyl or visible in a line, caliper or wheel cylinder presents itself with a pedal that either is firm (with or without pumping), but fades (as fluid leaks) or a pedal that will not become firm with pumping.

As for the proportioning valve, i believe it should be part of your system, but leave that up to the experts here.

Best of luck and keep us posted!

[Ghost's68]

Originally Posted by joeminnesota

What components did you replace?

It is reasonable to suspect the last replaced component if the new symptom occurred after that installation.

If none of the new components are faulty, i would replace the Master Cyl.

Air in the system is indicated by a soft pedal that when pumped a few times becomes firm, after all, air is compressible. A leak, weather internal to the Master Cyl or visible in a line, caliper or wheel cylinder presents itself with a pedal that either is firm (with or without pumping), but fades (as fluid leaks) or a pedal that will not become firm with pumping.

As for the proportioning valve, i believe it should be part of your system, but leave that up to the experts here.

Best of luck and keep us posted!

I just replaced rotors, calipers brake lines and pads. I don't see any leak or drops from the brake system.

If I swap the M/C do I need it for the 73-87?

Sorry for no further updates. I just got the wheel alignment done and just got the truck back. Haven't worked on it for a while.

[joeminnesota]

Not sure on which master cylinder. I don't know the differences (if any) between a 72 and 73..up. Maybe one of the experts here can help.. If all else fails take yours to the parts store and match it to a 73...then a 72.. ugh..no fun. good luck.

[Wrenchbender Ret]

If your parking brakes are part of the service brakes they often stick & don't adjust themselves. This causes a low pedal. Take off the rear wheels & see if the pads are up against the rotors. If they are away from the rotors that is your problem.

[Ghost's68]

I finally got around to taking some pics of my booster/MC combo. The mounting doesn't look very correct as you can see from the use of the washers and only 3 out of the 4 holes being used. The bracket should sit flush against the firewall I believe. Can anyone give me some pics of how it should be installed? Thanks guys

[zosoppp]

see my pic in post 10. There should be a bracket with a "z-bar" or bellcrank linkage between the firewall and the booster. the bellcrank would be between the pedal rod and the booster rod.

[demian5]

looks like someone did a crappy manual to power conversion. Some scary firewall stuff going on there. I just replaced my booster on my factory power brake truck and yes, like said earlier, it has a bell crank that basically puts the end of the booster at the top of the hole going to the brake pedal.

And that master is a drum/disc master. You need a corvette master for 4-wheel disc. So a 74 master from a power brake vette would work fine.

I would be looking at CPP for the right stuff so you don't die.

If it were at a shop they would make you sign a release stating that if you die they are not liable...

[demian5]

http://www.classicperform.com/Store/...ck/6062BB9.htm

The image on the right has it all including the bell crank.

[67 chevelle]

Originally Posted by demian5

looks like someone did a crappy manual to power conversion. Some scary firewall stuff going on there. I just replaced my booster on my factory power brake truck and yes, like said earlier, it has a bell crank that basically puts the end of the booster at the top of the hole going to the brake pedal.

And that master is a drum/disc master. You need a corvette master for 4-wheel disc. So a 74 master from a power brake vette would work fine.

I would be looking at CPP for the right stuff so you don't die.

If it were at a shop they would make you sign a release stating that if you die they are not liable...

I agree , plus I would add a "combination valve" so you have a brake warning light , it could save you, or someone elses life .

[Ghost's68]

Originally Posted by demian5

http://www.classicperform.com/Store/...ck/6062BB9.htm

The image on the right has it all including the bell crank.

Originally Posted by 67 chevelle

I agree , plus I would add a "combination valve" so you have a brake warning light , it could save you, or someone elses life .

Thanks a lot guys for the help! That CPP link to that booster is what I need? If it is, I'll order it tonight. As far as the M/C I'll look into the corvette one as well. Thanks again guys!

[Ghost's68]

Just went on the CPP site and found this disc/disc combo for the 67-72. Would this kit be what's needed? It looks like it comes with the entire bracket as well.

http://www.classicperform.com/Store/...ck/6772BBD.htm

[demian5]

Originally Posted by Ghost's68

Just went on the CPP site and found this disc/disc combo for the 67-72. Would this kit be what's needed? It looks like it comes with the entire bracket as well.

http://www.classicperform.com/Store/...ck/6772BBD.htm

You have achieved rockstar status. Please report back. And make sure the check valve works properly. I bought a kit for my impala and the booster "honked" when you pressed the brake pedal. The check valve wasn't completely opening.