Disc brakes not stopping?

[amberkalvin]

I've researched and have built a good handful of trucks and have done tons of brake jobs. However, this one has me stumped...

I have something that looks like this kit

And it seems no matter what I can't get enough pressure to stop the truck. I've ruled out vacuum since this truck has a large cam and went ahead and put a $230 vacuum pump as well as a vacuum canister that is supposed to pull at least 18" of vacuum. The engine only pulls 12". I can leave it for 10 or 20 mins and pull a vacuum line off and hear a bunch of pressure release so I don't imagine the booster is bad.

All rubber lines are new, freshly turned rotors, new pads and brand new calipers. Properly bled with a firm pedal. Brand New master cylinder off of a 74 c10 as I thought it might've been bad, plumbed to match my K5 which stops like a champ and a brand new proportioning valve.

My thought is this. With the setup I have now, I am losing leverage by the pivoting portion of the booster bracket. Whereas this kit listed below is straight from the pedal to the booster.


https://www.summitracing.com/parts/c...EaAh3iEALw_wcB

Any help would be greatly appreciated. I have never run across this kind of issue.

[geezer#99]

Did you change your pedal ratio to 4.5 to 1?

[amberkalvin]

Originally Posted by geezer#99

Did you change your pedal ratio to 4.5 to 1?

I did not do anything with the pedal. The truck came to me with the disc brake conversion, how do I go about checking and ensuring it's the correct ratio? I did look under the dash and it looks like there is only the one place to hook the booster to.

Thanks for the quick reply.

[geezer#99]

Measure from the pivot point at the top of the pedal down to the bottom of the pedal. Don’t worry about the curve of the pedal. Then measure from top to hole rod is hooked to. Divide that into your total length to get the ratio. Stock ratio is usually 6 to 1.
Usually you need a hole drilled about 1 1/4 inches lower than stock to get 4.5 to 1.

[amberkalvin]

Originally Posted by geezer#99

Measure from the pivot point at the top of the pedal down to the bottom of the pedal. Don’t worry about the curve of the pedal. Then measure from top to hole rod is hooked to. Divide that into your total length to get the ratio. Stock ratio is usually 6 to 1.
Usually you need a hole drilled about 1 1/4 inches lower than stock to get 4.5 to 1.

Okay, so in my thinking, it was that I didn't have enough leverage, however is it instead that I'm just not getting enough travel? Drilling a hole lower would lessen the amount of leverage I have but I assume if it has more movement might help push more fluid to the calipers and what not?

Thanks again for helping out!

[Furno68C20]

I hear you! I recently converted my 68 c20 over to front disc along with new booster/ master and prop valve. The problem I had was bleeding the rear circuit. I wasn’t building pressure out of the master into the prop valve and then to the rear. I found that the new master was part of a “bad” casting batch from the supplier. The ports out of the master leaking during bleeding hence no pressure. Check for leaks would be first on my list since these castings are out there.

Hope that helps. Keep us posted!
Posted via Mobile Device

[geezer#99]

Originally Posted by amberkalvin

Okay, so in my thinking, it was that I didn't have enough leverage, however is it instead that I'm just not getting enough travel? Drilling a hole lower would lessen the amount of leverage I have but I assume if it has more movement might help push more fluid to the calipers and what not?

Thanks again for helping out!

Yup! You got it.
More travel needed.

[amberkalvin]

Originally Posted by geezer#99

Yup! You got it.
More travel needed.

Dude. You are awesome. I'll try to get this knocked out tomorrow and update.

I love this forum!

[amberkalvin]

Originally Posted by Furno68C20

I hear you! I recently converted my 68 c20 over to front disc along with new booster/ master and prop valve. The problem I had was bleeding the rear circuit. I wasn’t building pressure out of the master into the prop valve and then to the rear. I found that the new master was part of a “bad” casting batch from the supplier. The ports out of the master leaking during bleeding hence no pressure. Check for leaks would be first on my list since these castings are out there.

Hope that helps. Keep us posted!
Posted via Mobile Device

Yea I'll double up on checking for leaks as well! Thanks!

[geezer#99]

Hopefully that works.
If not there’s a couple more things to look at if we need to.

[amberkalvin]

Originally Posted by geezer#99

Hopefully that works.
If not there’s a couple more things to look at if we need to.

Well, no dice. I got to thinking and wasn't sure this was going to help just because the pedal is so firm to begin with. It feels even harder to push and the same amount of travel. Maybe it's a bad new booster or something? Everything else is branad brand new, the booster is new, but has been on there a while and the only thing I haven't replaced. Or again maybe it's the pivoting arm leverage loss?

[geezer#99]

Check the pushrod that’s between the booster and the master.

I assume you’re running your booster off a line plumbed into the vac can.

[webfoot]

If your booster is bad the idle will increase with the brake pedal in. You should also have some assist left once the engine is off until you depress the pedal a few times and let all the vacuum out with a good booster.

I had a reman fail after 2 or 3 years of only occasional use and upgraded to a later style.

I'm guessing you still have air in the lines, somewhere.

[amberkalvin]

Originally Posted by geezer#99

Check the pushrod that’s between the booster and the master.

I assume you’re running your booster off a line plumbed into the vac can.

The vacuum pump is routed to the vacuum canister which is routed to the vacuum booster. Like I said I don't think there's any leaks as I can walk up 20 mins later and pull a line and hear it depressurize.

Ill check and double check the pushrods situation.

[amberkalvin]

Originally Posted by webfoot

If your booster is bad the idle will increase with the brake pedal in. You should also have some assist left once the engine is off until you depress the pedal a few times and let all the vacuum out with a good booster.

I had a reman fail after 2 or 3 years of only occasional use and upgraded to a later style.

I'm guessing you still have air in the lines, somewhere.

The vacuum booster is only hooked up to the vacuum canister and vacuum pump. Engine is not connected in any way.

The guy put a brand new booster master cylinder combo on then let it sit for 6 months. I honestly didn't think the booster was bad since it doesn't lose vacuum but I guess it could be seizing.

Not sure about air. Seems like it's bled pretty well.

[amberkalvin]

It almost seems more and more like the booster, like I said the pedal is firm, really firm, and it doesn't seem like there is any power brakes at all. Just brakes.

[Chris_oz]

Originally Posted by amberkalvin

It almost seems more and more like the booster, like I said the pedal is firm, really firm, and it doesn't seem like there is any power brakes at all. Just brakes.

You can test the booster before you replace it.

Pump the pedal several times (engine / vac pump off) to ensure no vacuum is in the booster.

Press firmly on the pedal and hold it down.
Start the engine / vac pump.
You should feel the pedal slowly fall as the vacuum increases and helps boost the effort.
It is easier with an engine supplying vacuum as the generally build faster, but the pump should do the same.

Do you have check valves between the booster/tank and the tank/pump?
Are they working as they should?
Many pumps wont hold vacuum when not running, so if you dont have a check valve between the pump and tank, you may have LONG recoery times and poor performance.

[amberkalvin]

I also found another thread that had the same exact issues as me, replaced all rear brake stuff because of seizing and all of a sudden it felt like there was power again. I'm gonna bite the bullet and replace it all. Can't hurt. Rear brake hose and hardware and shoes. Might as well at this point.

[amberkalvin]

Originally Posted by Chris_oz

You can test the booster before you replace it.

Pump the pedal several times (engine / vac pump off) to ensure no vacuum is in the booster.

Press firmly on the pedal and hold it down.
Start the engine / vac pump.
You should feel the pedal slowly fall as the vacuum increases and helps boost the effort.
It is easier with an engine supplying vacuum as the generally build faster, but the pump should do the same.

Do you have check valves between the booster/tank and the tank/pump?
Are they working as they should?
Many pumps wont hold vacuum when not running, so if you dont have a check valve between the pump and tank, you may have LONG recoery times and poor performance.

I'll double check the vacuum booster TMR, but I have yet to feel any type of booster assist. There is actually two check valves, one on the booster and one on the canister so there is quite the reserve and it seems to hold vacuum fine.

Good point though. It should be boosting brake pressure somewhere along the line. I'm not getting any at all.

[Steeveedee]

Somewhere in the mix, I have found that some master cylinders use the front part of the reservoir for the front brakes, and some use the rear. In any event, you may want to check that the fluid is routed properly to the front and rear brakes. If they are switched, you will have poor brake performance, even with everything else done right. How to check that, I'm not sure. But I've seen where some people have had to fiddle with the lines. Don't seem to be able to find a thread about it, though.

[Sheepdip]

Originally Posted by Steeveedee

Somewhere in the mix, I have found that some master cylinders use the front part of the reservoir for the front brakes, and some use the rear. In any event, you may want to check that the fluid is routed properly to the front and rear brakes. If they are switched, you will have poor brake performance, even with everything else done right. How to check that, I'm not sure. But I've seen where some people have had to fiddle with the lines. Don't seem to be able to find a thread about it, though.

The big reservoir is for the disc and the small for the drums. Disc brake pistons do not return like drum shoes so they eventually wear and need more of a fluid reserve, especially if you do not check/service and top it off.

If both reservoirs are large they are known as a Corvette Master and are designed for 4 wheel disc brakes, they can be used on a disc/drum with no issues.

[jkeady]

I would start with the cheap and easy: tee into your booster vac line with a vacuum gauge and verify that you have as much vacuum as you think both before and after you depress the brake pedal.

[webfoot]

Originally Posted by amberkalvin

The vacuum booster is only hooked up to the vacuum canister and vacuum pump. Engine is not connected in any way.

The guy put a brand new booster master cylinder combo on then let it sit for 6 months. I honestly didn't think the booster was bad since it doesn't lose vacuum but I guess it could be seizing.

Not sure about air. Seems like it's bled pretty well.

Hook it up directly to engine vacuum and test.

If you have no air in the lines and no assist, you'd still have brakes assuming you can put enough leg into it.

[RichardJ]

>>Disc brake pistons do not return like drum shoes so they eventually wear and need more of a fluid reserve, especially if you do not check/service and top it off.<<

So, for the purposes of trouble shooting what the brakes are doing today, it really doesn't matter if the MC runs out of fluid five years from now.

>>If both reservoirs are large they are known as a Corvette Master and are designed for 4 wheel disc brakes, they can be used on a disc/drum with no issues.<<

That's hardly acceptable logic. My Corvette has a single reservoir that feeds front and rear of the MC. Most brake systems on newer vehicles, have low-fluid level warning devices so it doesn't matter if the reservoir is too small !




>> the cheap and easy: tee into your booster vac line with a vacuum gauge <<

What is easy about hooking up gauges??????

Originally Posted by Chris_oz

You can test the booster before you replace it.

Pump the pedal several times (engine / vac pump off) to ensure no vacuum is in the booster.

Press firmly on the pedal and hold it down.
Start the engine / vac pump.
You should feel the pedal slowly fall as the vacuum increases and helps boost the effort.

What could possibly be easier than pushing on the pedal ???????

amberkalvin, follow the instructions from Chris_Oz. All the rest of this crap is just that, until you determine if you actually have boost or not.

[RodnRudy]

I had a similar problem with the hard brake pedal at start up on my 68 with power disc brakes. When I would start the truck i'd have a hard pedal and brakes wouldn't hold the truck. I have 17" of vacuum at the booster. After about three to five minutes all of a sudden the brakes would be fine. Come to find out the check valve on the booster was sticking and it would take a while for it to free up. This was on a brand new booster from CPP. I replaced the check valve with one from NAPA and viola problem solved!
Try changing the check valve.