Wrong master cylinder?

[87Skier]

Looking some info...

I thought that these came with either of two or three different brake setups. I thought that they had a smaller diameter thicker booster, a larger diameter thinner booster, and a manual setup. I also thought that dependent on what booster you had is what master cylinder you had.

I have a large diameter thin brake booster, but I believe that I am running the wrong master cylinder. I replaced it about a year ago, when I redid my entire brake system before I started driving it. I bought a new one at a trusted parts store, and it was identical to the one that came off there.

The problem I am having is that the rear brakes feel like they are the only ones working. If I'm on the highway and hit the brakes hard, the rear brakes lockup, the truck gets squirrely (even when the rear don't lockup) and the brake pedal is very hard. I'm 99% sure that I have a drum brake master cylinder and not a front disc master cylinder.

Can anyone confirm this?

[bamachem]

That looks like the same master cylinder and proportioning valve I have on my 72 blazer and it performs in a similar manner. The driver rear will lock up pretty quick on you if you have to hit the brakes for anything other than a "normal" stop.

[Jurassic-1]

Could the proportioning valve be off center inside? I purchased a small tool from Brothers which is said to aid in keeping the prop valve centered when bleeding.

[87Skier]

It's a possibility, but I feel like it has to be something more related to the master cylinder itself because of the very hard brake pedal.

I'm thinking if I swap mine out for a master cylinder that has a very small and very large reservoir it will apply the front with a lot less brake pedal force. I was hoping someone had this style on theirs and could confirm this.

[Jurassic-1]

Is that master cylinder equal size on both reservoirs ? If it is it could be like the Corvette Master Cylinder for 4 wheel disc which would be the wrong one. The hard pedal could also be a failing brake booster.

[pound41]

changing the brake booster fixed mine

[viking427]

You need a disc/drum MC and proportioning valve, or your rears will lock up before your fronts - never good unless you enjoy fishtailing up to someone's rearview mirror. The disc/drum MC's have a larger reservoir in the front, since calipers need more fluid to operate properly. The vacuum boosters are not as critical, as long as you have the correct length MC push rod/button to match the booster's plug depth.

[87Skier]

Originally Posted by pound41

changing the brake booster fixed mine

Which issues did you have?

I went ahead and ordered a new master cylinder. I should be able to change it out this weekend, and will let the rest of you know. I'm thinking that someone changed it out for the wrong one at some point and left the proportioning valve alone.

[ls1nova71]

That picture you posted is NOT a drum brake master cylinder, it is definitely a factory '71-2 truck disc/drum master, and the proportioning valve looks correct too. Sounds to me like you may have a proportioning valve issue. But if you have a hard pedal, it could also be a booster problem. Are you sure the booster is working? You can test it by pumping the brakes with the engine off to release any stored vacuum, then with your foot on the brake, start the engine, you should feel the pedal move down a bit when the assist occurs.

[87Skier]

The brake booster definitely works.

It is difficult to describe the exact pedal feel but I'll try my best.

Under normal driving conditions the pedal feels just like any other vehicle. It is firm providing good feedback, but not to firm indicating that the booster is working.

Under hard braking situations the pedal has two stages it goes through. When you first push the pedal it feels normal, but you aren't stopping anywhere as fast as you like to be. At this point it feels you are pushing on a brick wall with your foot, like the brakes are fully engaged. If you hold your foot here long enough the front brakes suddenly grab and begin slowing how you initially thought they should be.

Under emergency braking situations it feels really strange. On gravel or any other slippery surface, the rear lock up immediately. Then you skid to a stop while trying to keep the back from overtaking the front. On a sticky surface like asphalt, when you hit the brake as hard as you can, the rear again lock up, the front never do. If you hold it like this long enough the pedal then travels furthers while maintaining CONSTANT pressure.

My best guess at what is happening is that the front aren't getting enough pressure. The slight play in the pedal and the initial travel is the rear brake cylinders expanding, then starting to work normally. The hard point in the pedal is where the front start working, but they don't have enough pressure. It really makes be think that the bore size is wrong, or that something is up with the proportioning valve.

According to the service manual the proportioning valve (Brake Combination Valve), it is supposed to allow pressure to build for the rear brakes, then at this point open the front brake valve while regulating pressure. I suppose its possible that the valve is improperly adjusted or just stuck.

I should mention that I replaced the front pads, calipers and hoses, master cylinder, rear axle hose, and rear wheel cylinders. We don't salt our roads here so no reason to replace the brake lines, and they bled fine. Should I replace my proportioning valve?

[viking427]

ls1n is probably right - the earlier dual chamber disc/drum MC's do look very similar to drum/drum MC's. The rear drums fluid circuit was operated via the front chamber and the front disc fluid circuit via the rear chamber - opposite of what eventually became adopted as industry std.

Heres a couple links that may help visualize better;

http://67-72chevytrucks.com/vboard/s...d.php?t=451659

http://outintheshop.com/faq/Brakes/brakeinfo.html

http://outintheshop.com/faq/Brakes/propvalve.html

http://www.classicperform.com/TechBo....htm#testpropl

[87Skier]

So what would happen if I bought a newer style master cylinder and did away with the proportioning valve? Like the one below.

Also, according to the http://www.classicperform.com/TechBo...htm#testmaster my proportioning valve is working correctly. It could still be stuck though.

[viking427]

Keep your prop valve, you need it. That newer style uses one too (albeit likely in a combo valve), but its just not shown in the pic.

Check your plumbing - lines out of your MC to prop valve, out of prop valve to metering/distrib block, out of metering/distrib block to front & rear brakes. Check the "rear brakes" port on your MC (front chamber/line out on your older style MC) - look directly into this threaded outlet port with flashlight and magnifying glass - you should see a small tapered brass insert with tiny rubber plunger hiding inside there - this is your 10 psi rear drum residual valve. The Disc chamber outlet port should not have one. We used to pop these out with a drywall screw when converting drum MC's over to a 4 wheel disc MC's.

Check your booster to make sure the "button" (short steel pushrod) is in there sandwiched b/n your MC and vacuum booster - it needs to be the short version for power brakes, not the longer one. Make sure your booster isn't leaking vacuum when pressing the brake pedal - you'll normally hear a slight
"hissing" if so.

Open up those links and spend some quality bedtime reading to understand braking systems and components. It'll help you troubleshoot brake systems now and in the future. Otherwise a brake shop can probably troubleshoot your older more basic system in 20 mins.

[87Skier]

I'm pretty familiar with how they work. I read those articles and understand how they work.

When you say "check" are you saying open up the lines and run something through to make sure they are clear or just check for kinks/leaks?

[viking427]

"check" = look at

[Ole Beater]

Could the rear brakes be over adjusted?

[87Skier]

I got around to checking everything out and couldn't find any issues with the brake lines. They all appear to be in good condition, no rust or kinks in them.

The only thing I did find that could be an issue is that the button on the front of the proportioning valve seems to be stuck. I've heard you have to hold this while bleeding the brakes. Any one had any experience with this?

[71jimmy]

does the "brake" light on your dash come on and stay on? This would indicate there is an issue in the brake circuit, front to back pressure etc.

There is a special tool (I made my own) used to hold the button in while you bleed the brakes. The factory service manual has a pic of it.

It took me several bleeding cycles to get that darn light on the dash to finally stay off. If the switch is bad, don't know if you can just replace it or need to replace the entire proportioning valve.

[Chulisohombre]

Sounds like you need a adjustable prop valve. A lot of guys need one for 4 wheel disk. But in this situation you could back off the pressure to the rear with the dial and get the fronts to brake faster than the rears.

[87Skier]

No light on dash except when cranking. Also verified the circuit by grounding out the wire.

Where can I get an adjustable prop valve? I'm guessing I will need to change up the lines?

[Chulisohombre]

I think wilwood makes them.

[chadcampbell00]

I had this exact problem with mine. I finally bypassed my factory distrubution valve on the side of the master cylinder and put an adjustable proportioning valve in the line going to the rear brakes allowing me to decrease pressure to the rear. It stops 100xs better now. My plan is to get a new distrubition valve and put it back to normal but i haven't gotten around to that yet.