Brake problem question

[Mdavis83]

I just picked up a 72 and it has a brake issue. When you use the brakes at first pressure they are soft and then they just about throw you through the windshield. The truck has sat for the last 2 years before I got it but I believe the previous owner had the same issue before it was parked. It has factory power brakes with front disk rear drum. I know the master cylinder is leaking but I don't think that is the issue. I'm thinking it is possibly an issue with the prop valve or maybe just the slides for the front calipers hanging up and then sliding all of a sudden causing the soft pedal, then hard braking. The truck does have a comp xe 268 cam, but I don't think it is radical enough to cause vacuum issues. Still trying to figure everything out on this truck. Any thoughts or suggestions?

[everettelane]

Try to bleed the brakes check for rust when bleeding if the master cylinder is leaking inside the truck the seals are probably shot on the piston mine was full of rust also my front calipers were sticking so i changed everything including the lines lol!!! the brakes are the last thing you want to have problems with.

[one3n3my]

I had the same issue with my 70. Check the prop valve and the lines as it could be losing pressure there. Also check the booster, I replaced it twice before it was functional.

I had come to find out the previous owner had replaced the line with a larger diameter which was causing the rear breaks to not receive the pressure that it needed to engage.

I replaced the booster, master, brakes, and lines in order to figure it out.

[Mdavis83]

I am definitely going to replace the master. Kind of want to just replace the whole assembly. What is a good booster/mc/prop valve kit to buy that isn't ridiculously expensive?

[Mdavis83]

Originally Posted by Mdavis83

I am definitely going to replace the master. Kind of want to just replace the whole assembly. What is a good booster/mc/prop valve kit to buy that isn't ridiculously expensive?

Anybody have a suggestion??

[Mdavis83]

I was thinking maybe part number SUM-760197 from summit and I would have to buy a new prop valve to correct? Tried to post the link but it didn't work right.

[Mike_The_Grad]

I just redid my brake system on my 1972 c10 after a failed passenger side wheel bearing decided for me. New rotors,pads lead to new rubber hoses,leading to new brake booster,leading to new master cylinder,leading to new front brake lines...eventually it'll be all new. I just went with over the counter reman brake booster and new ac/delco master cylinder. One thing I was told and had in my head from the beginning is, brakes are definitely not something you want to save pennies on. You should always try to buy "new" vs. "Reman" anytime you can.Stainless brake lines are one of those "I can afford it." Items. They sure do look nice and practically last forever, but are a PITA to have to mess with when things don't wanna route where they need to be.

[Mdavis83]

Originally Posted by Mike_The_Grad

I just redid my brake system on my 1972 c10 after a failed passenger side wheel bearing decided for me. New rotors,pads lead to new rubber hoses,leading to new brake booster,leading to new master cylinder,leading to new front brake lines...eventually it'll be all new. I just went with over the counter reman brake booster and new ac/delco master cylinder. One thing I was told and had in my head from the beginning is, brakes are definitely not something you want to save pennies on. You should always try to buy "new" vs. "Reman" anytime you can.Stainless brake lines are one of those "I can afford it." Items. They sure do look nice and practically last forever, but are a PITA to have to mess with when things don't wanna route where they need to be.

Thanks for the reply. I definitely don't want to cheap out on the brakes,just don't have the budget to spend a ton of money on a booster/master/prop valve kit. I don't mind spending a little bit tho. Have looked at the cpp kit but I'm a little skeptical with all their bad customer service reviews. The main thing is I want to get something that fits and works properly without any headache. If I have to do new lines, I work as a tuning bender by trade so I could make my own. Right now I just want to get them working properly without putting me through the windshield lol

[Mdavis83]

Did you replace the proportioning valve also? It seems those are somewhat difficult to find.

[Ruscal]

Getting the leaking MC replaced is the best first step. The proportioning valve would be good to do at the same time. Flush all the old fluid out and check how bad it looks. If you see a lot of rust it would be a good idea to replace the lines and hoses. Money being tight, do the front system first, then the rear at a later time. You should get a firm pedal. If your first push of the pedal is soft, it means there is air in the system.
If your rotors are rusty from being parked it could cause the grabbing. Low vacuum would not cause over-sensitivity.
Russ

[kwmech]

Where is the master cylinder leaking? First thing you do is put the truck up on stands and pull all 4 wheels and take a look. Grabby brakes can be a leaky wheel cylinder ( brake fluid leak MC) or axle seal. You could have a partially stuck wheel cylinder (rust)

[Mdavis83]

Originally Posted by kwmech

Where is the master cylinder leaking? First thing you do is put the truck up on stands and pull all 4 wheels and take a look. Grabby brakes can be a leaky wheel cylinder ( brake fluid leak MC) or axle seal. You could have a partially stuck wheel cylinder (rust)

I'm not 100% sure where it's leaking from. I haven't had a whole lot of time to mess with it. I've seen it drop off the front of the mc. It also has fluid where it connects to the booster.

[Mdavis83]

Had a chance to look around for a few mins and found that there was a broken wire that looks like it goes to the rubber piece on top of the prop valve. Could that has some effect on the issue?

[Mdavis83]

I soldered the wire back together and the brake light on the inside gauge just stayed on.

[Mdavis83]

Bump

[Crabass]

As "Mike" advised, brakes are one area you do not skimp on, and to that I'd add that you shouldn't be working on unless you absolutely know what you are doing. No offense, but I'd advise that you find someone that knows these systems.
From your descriptions I get a sense that you may have a mismatched component issue. I dealt with a very similar situation on an old C3 Corvette, pretty much the same symptoms. Turned out the pushrod and m/c were mismatched.
There is no telling what some PO has cobbled together in the past. Just because something bolts together doesn't guarantee that it ought to. "One3n3my" mentioned just such a situation with his rig. He ended up with a new brake system and I doubt, in the end, he ever regretted having that.

[crakarjax]

If your brake light is on, that means the brake warning switch is engaged. That means that there's been a large pressure differential between your front and rear brake circuits. If it were me I'd take the prop. valve out and disassemble, clean and reassemble it -- may be crusty from sitting for years. Then reassemble and gravity bleed the system until new fluid is coming out on all 4 corners.

When the brakes grab does it nose dive? If so it's probably you front brakes that are grabbing. The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.

Anyway, get a new MC, they are cheap. Just get the stock MC, boosters are expensive and based on your description it's not the issue at all, it's just there to make pushing the pedal easier. Bench bleed the MC, clean out the prop. valve, bleed the brakes, make sure the front discs look clean etc. and if you still have problems after all that... well idk, you probably won't!

[Aus69c20]

Great info crakarjax.
I didn't know about the rear brakes being applied first in light braking.

[demian5]

When you unplug the newly soldered wire, does the light on the dash turn off? If yes, you have an issue with the pressure differential between the front and rear brake circuits as crakarjax mentioned.

[Mdavis83]

Originally Posted by crakarjax

If your brake light is on, that means the brake warning switch is engaged. That means that there's been a large pressure differential between your front and rear brake circuits. If it were me I'd take the prop. valve out and disassemble, clean and reassemble it -- may be crusty from sitting for years. Then reassemble and gravity bleed the system until new fluid is coming out on all 4 corners.

When the brakes grab does it nose dive? If so it's probably you front brakes that are grabbing. The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.

Anyway, get a new MC, they are cheap. Just get the stock MC, boosters are expensive and based on your description it's not the issue at all, it's just there to make pushing the pedal easier. Bench bleed the MC, clean out the prop. valve, bleed the brakes, make sure the front discs look clean etc. and if you still have problems after all that... well idk, you probably won't!

Yes the front nose dives. I am definitely going to replace the MC and Booster. I'm pretty sure the MC has been leaking fluid into the booster. When I'm at a stoplight for any length of time the truck starts creeping and I end up with the brake pedal on the floor. If its on the floor for long I have to let off and reapply so I'm pretty sure the booster is going bad. I'm just not sure if I want to replace the prop valve or not. My dilemma is if I should buy the whole MC/booster/prop valve kit or just replace the booster and MC and try to clean the prop valve. It's about $300 for the kit or I can spend about 150 and reuse the prop valve. I'm hesitant to order the kit because if it doesn't fit right I have to deal with return shipping, etc. I mainly just want to get parts that fit right so I'm leaning towards buying a MC and booster local. This isn't outside the realm of my ability to work on. I've done brakes several times before, just haven't dealt with a problem like this, and I haven't dealt with a prop valve. But that's what forums and youtube are for right?! Thanks for all the info.

[Mdavis83]

Originally Posted by demian5

When you unplug the newly soldered wire, does the light on the dash turn off? If yes, you have an issue with the pressure differential between the front and rear brake circuits as crakarjax mentioned.

Yes it does. Also, I'd like to throat punch whoever cut the wire so close to the plug boot.

[zosoppp]

If the truck starts creeping forward while you are holding the pedal down, I'd think it's the MC leaking fluid back to the reservoir rather than a power booster issue.

Or any other leak from the brake lines/slave cylinders/calipers too.

[Mdavis83]

Originally Posted by zosoppp

If the truck starts creeping forward while you are holding the pedal down, I'd think it's the MC leaking fluid back to the reservoir rather than a power booster issue.

Or any other leak from the brake lines/slave cylinders/calipers too.

That may be the case, but it's worth the extra $100 to me to know it's not gonna be an issue. There is fluid where the mc bolts to the booster so I can only assume it has leaked into the booster. I'd rather replace it along with the mc, than to have to pull the mc back off again.

[demian5]

Make sure that combo valve isn't stuck. Ive seen a couple go bad from a bad master.

[Mdavis83]

Originally Posted by demian5

Make sure that combo valve isn't stuck. Ive seen a couple go bad from a bad master.

Combo valve??