Brake pedal very hard on C10 1971

[syncrov69]

I rolled my c10 1971 for the first time today since 2007. I finished installing the brakes today. The brakes work but the pedal is really hard and I have to push the pedal with my 2 foots to brake. Lines, calipers, wheel cylinders and master cylinder are new. The booster been check and should be functional, one piece that I have not been able to verify is the proportioning valve. I also installed a proportioning valve and master cylinder for a C30. What could my problem be related to pedal too hard? Thank's for your input and sorry for my bad english

[dragonwrecker]

ck the vacuum on the booster

[syncrov69]

Thank's I try this today! What is the minimum vaccum necessary for the booster?

[68gmsee]

Actually, there's several things that can cause the problem including any of the new parts.

First check the booster as dragonwrecker mentioned and remember that new parts do not necessarily mean good parts -especially these days with a lot of parts made or rebuilt in other countries. You can google "testing the brake booster" and look for instructions on how to check it out.

If the booster checks out, there's a possiblity the proportioning valve is malfunctioning. As you know, it's supposed to proportion the fluid between the front and rear brakes and if it allows more fluid to the rear, it will be harder to stop the vehicle and in some cases lock the rear wheels.

The master cylinder has two pistons -one for front and the other for the rear brakes. One of the pistons could possibly have an internal leak preventing fluid to get to the brakes.

Other possiblities are restricted brake line hoses -one on each side in front and another above rear axle. If they're old, replace them. They swell internally and the failure is not noticeable.

Last is contaminated or cheap brake shoes. Check each one of the problems with the easiest to check (booster) first and let us know what you find.

[syncrov69]

Hello I did the check of the brake booster this morning and I have a lot of vaccum coming from the engine and I unplugged the one way valve on the amplifier and after 1 minutes of engine shutdown and was plenty of vaccum again. My problem does not seem to come from the amplifier! In terms of metal lines and hoses are all new and no apparent leaks. But I can still brake but pressing the pedal with 2 feet of all my strength!

[syncrov69]

This afternoon I changed the Proportioning valve to try another Proportioning valve that I had in my inventory of used parts. It's always the same as before the pedal is hard and does not go down!

[k2rm]

When you bled the brakes, were you able to push the pedal down easily? Were you able to get fluid out of all of the bleeders?

If so, then your MC is likely OK and you should focus on your booster. Is your check valve working?

[syncrov69]

Yes when bleeding pedal go bottom. Check valve is ok and keep vaccum in bosster after engine stopped! Fluid come easily from each bleeder!

[68gmsee]

Two feet to stop the vehicle is not right -and not safe. I still think your booster is suspect but also glazed over or cheap brake pads.

If you are sure the booster is assisting and m.c. is working properly, replace the front pads with quality pads and make sure the rotors are not burnt or glazed. If they are either have them turned if it can be done or replace them. It's the safest thing to do.

[dragonwrecker]

i dont think your problem lies in you pads. but before i can say that for sure i want to ask a question.is the pedal hard to press and/or moves very little with alot of force or does it press down properly but no stopping force at the wheels.

[dragonwrecker]

you said the mc is from a c30 if the master cylinder bore is too large, it will cause a very hard pedal feel with minimal pedal travel, and will have a difficult time generating the necessary pressure to effectively operate the brake system. If the master cylinder bore size is too small, the driver will be able to generate a lot of pressure, but the pedal may have an excessively long stroke or a very spongy feel.

[syncrov69]

Originally Posted by dragonwrecker

i dont think your problem lies in you pads. but before i can say that for sure i want to ask a question.is the pedal hard to press and/or moves very little with alot of force or does it press down properly but no stopping force at the wheels.

the pedal hard to press and/or moves very little with alot of force

[dragonwrecker]

ok then your problem is not in the pads it is most likely the master cylinder itself if booster has good vacuum

[slikside]

subscribed

[Christopher J]

Any resolve for this, or other suggestions?

I'm having the same problem on my '72 Suburban C20.

Just replaced the original brake booster and master cylinder with a new kit I bought from The Truck & Car shop in Orange, CA and the same problem described above persists that was happening before the booster and master were replaced with a new kit.

I'm about to pull the replacement kit so I can drive an hour back to the shop to replace it.

Originally Posted by syncrov69

the pedal hard to press and/or moves very little with alot of force

[Mike_The_Grad]

Originally Posted by Christopher J

Any resolve for this, or other suggestions?

I'm having the same problem on my '72 Suburban C20.

Just replaced the original brake booster and master cylinder with a new kit I bought from The Truck & Car shop in Orange, CA and the same problem described above persists that was happening before the booster and master were replaced with a new kit.

I'm about to pull the replacement kit so I can drive an hour back to the shop to replace it.

On my 1972 custom/10 350/th350 LWB disc upfront/drum rear.

I recently rebuilt my engine, have about 2000 miles on it. Took me about 15 months of weekend work only, I also rewired vehicle and put new brake fluid(bled properly),pads,shoes,vacuum line from base of carb. To booster all new w/new check valve and grommet. I have notice my brake pedal has to go 99% of the travel in order to feel like its working.my foot will travel led than an inch after that, basically putting my foot on the floorboard. If I let up that same amount I'm rolling again. And also when I'm depressing the pedal and near the end of the travel,my brakes will squeal/screech. I just topped off fluid this past weekend with rear reservoir about 1/2 inch below overfill. When truck is off I do not feel any resistance with brake Peel travel. (It feels the same whether truck is running or not.) One day while I was in process of rebuilding engine, I popped hood to reference something and had notice the reservoir had "burped" some fluid out and had about a 1 inch diameter spot on inner fender with a fresh drop still clinging to reservoir. I shrugged it off as thinking my brother may have forced my truck out of the way in order to get access to his motor cycle. If he had forced the truck in any direction with parking brake engaged,would fluid "relieve pressure" out of reservoir? Or even force its way past a seal of some sort? And finally, would the booster whistle or whine or make any audible sound if it were worn out or on the verge?
Lengthy, I know...but I'm facing issues with my recent build that I think I am slowly narrowing down to vacuum issues.

Any info is much appreciated I do own,service manual,factory assembly manual, and Chilton's service manual. Google is my goto, and this site has humbled me in more ways than I can count.(glad to know I'm not the first/only person with any issues these trucks like to throw our way! Lol)

[cdowns]

pushrod length is critical and should b set to factory specs as listed in the factory service manual for proper operation of the brakes