1981 GMC Sierra 3500 - Help with brakes

Brake pedal is low and must be pumped to stop vehicle. Pumping raises the pedal but the pedal drops again when not pumping. Brakes were bled but condition is not changed. There is a leak in the rear brake circuit as the small chamber of MC slowly gets lower with time. I blocked off both outputs of the MC and the pedal was high and solid. With the front line plugged only, the pedal drops an acceptable amount when applied. When the front line is not plugged, the pedal drops almost to the floor when applied and is spongy. Pedal is higher with engine off, but worse with engine running. There is no obvious evidence of a fluid leak anywhere. The fluid is dirty looking. I haven’t yet flushed out the dirty fluid. I plan to block off the brake lines at each wheel next and test the pedal height as I remove the block from each wheel. So far, this is all the experience I have troubleshooting a low brake pedal. I read a number of articles and watched a number of videos on topic. Any thoughts on this about what I should do?

You’ve already do a more than most before posting here. Clearly, your testing has shown these is a leak in the rear system. I will make the assumption you’ve looked carefully at the rear hoses and have pulled the drum looking for a leaking wheel cylinder with no finding.

I’d suggest you have a helper apply the brake and run the brake hard lines with your hands. When you find a wet spot, that is your leak. On a truck this old, the hard lines easily could have rusted to the point of a pinhole leak. I’d replace the entire line because you will find rusty patches at nearly every line clip, I’d bet. Good Luck!

Post back with what you find!

Remove the master cylinder from the brake booster and see if brake fluid is leaking into the booster.


The chamber that becomes low is the one farthest away from the brake booster. By the way, this truck has a hydrobooster. I don’t know if that makes any difference.

I haven’t yet removed the wheels. The rear brakes are drum brakes so have steel lines going to them, but there is a rubber hose about a foot and a half long that goes between the long steel line that runs from the combination valve at the front and the tee that’s bolted to the rear axle. This rubber hose looks very old and has surface cracks. I didn’t see any evidence of leaking. I will do as you suggest though, hopefully on the weekend.

I came up with an idea and hoping that someone can help with understanding how the system works and/or if I’m right in my thinking on this. I read in more than one article that if there are plugs in the master cylinder outlets and the brake pedal stays high, then the master cylinder is good. But I think this isn’t true. I think it’s still possible for at least one seal to be bad. As far as I know, the dual master cylinder is like two separate brake systems completely isolated from each other. That is, under normal operating conditions, the pressure from one of the two systems can’t effect the other system and fluid from one system can’t flow into the other system. So, I’m thinking that for example if the seal at the very front of the MC is bad, then the brake system for the other chamber should still be in good operating condition. If that one system is still in good operating condition, then it should hold the brake pedal high even though the other system has a bad seal.

Furthermore, because in my brake system, the pedal goes low, according to my theory, both systems (front and back brakes) must have a problem. I’m suspecting at least 3 problems: 1. a bad seal at the front of the MC (rear brakes), 2. a leak in the back brake system, and 3. either air trapped in the front brake system or a hose that expands but no leak.

Does any of this make sense?

Not exactly true. Both seals are driven by the pedal. If one of the 2 systems develops a leak, the pedal travel increases quite a bit so that at least the non-leaking system can apply the brakes. There is a complicated spool that carries those seals and allows a half-failure to still allow braking.

Pull the rear drums, now, not later. It is highly likely you have a wheel cylinder leaking. That is a far more common problem than a failed hard line. Considering this is a 37 year old truck, replace the rubber lines - front and rear - all 3 of them, they are dry rotted and will fail at the worst possible time. They won’t "balloon as you theorize, they burst. The inner liner of the hose is a nylon thread that doesn’t stretch like rubber. The rubber protects and seals.

Your theory is incorrect based on what I posted about how a dual master cylinder spool works. If both ends were leaking BOTH reservoirs would drain.

You can’t see a leak at the front, you haven’t completely checked for leaks at the rear. I’d bet a $100 without even seeing the truck that a rear wheel cylinder is leaking and filling up those huge drum brakes. If the linings got wet with brake fluid, change those, too.

So, to summarize. Order 3 brake hoses and 2 new rear wheel cylinders. Replace all those parts, inspect the metal brake pipes for rust damage and re-fill and bleed. If you really want piece of mind, replace the master at the same time. All your diagnostic checks may have damaged the MC seals.

What I meant by “no obvious” leaks was I wasn’t able to see any evidence of leaking on the parking space floor, on the outside of the tires and drums or on the lines and hoses (without removing the wheels). The truck belongs to my neighbor and my access to it is very limited, plus I have severe insomnia and I’m not feeling good much of the time. I took a look at the hoses on the front last night and they appear to be fairly new. The one on the back looks very old. Hopefully tonight I’ll get an opportunity to lift up the back and check the wheel cylinders. If not tonight, then I’ll check it out sometime in the next few days for sure.

One thing about web Forums is that people can post almost anything. So here I go.

This is your neighbors vehicle and you are on the web asking for help with brake repair. What if you actually do get them to work and then they fail and your neighbor is injured or someone else is you are a prime candidate for a very large lawsuit. This a case of no good deed goes unpunished.

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Do you suspect that I’m not competent enough to make sure they are safe when I’m done?
Do you suspect that I don’t care about my neighbors well being?
Do you suspect that I have no experience at anything mechanical?
Can a person with no money be sued?
Thanks for your input.

Every time I’ve encountered a leaking wheel cylinder, it’s run down the backside of the tire. Not saying it can’t appear on the front, but I haven’t seen that happen (yet). So laying on the ground and looking at the inside of the tires for evidence of liquid stains would be something I’d suggest that can be done fairly easily. Applaud your efforts to help out a neighbor…



What are your answers to my other questions?

Brake fluid has a special odor that you can easily detect.I find leaks 90% of the time with my nose.

I did check the backs of the tires. There’s no fluid leaking out of the drums, at least nothing showing. I’ve been under the truck and have looked over the entire system without removing the wheels yet. It’s a one ton so there’s lots of room under there. I can sit down under there by the rear axle.



Check if you can find evidence of brake fluid behind the wheel cylinders boots.

I appreciate your concern. Safety is important for me. I’m not a mechanic but I’ve worked on my own vehicles since I was a teenager, so more than 35 years. I didn’t work on other people’s vehicles - only my own. When I was working there wasn’t much time left in the day and because I had the money to spend, I usually got a mechanic to repair the more difficult problems. Sometimes I’d pay a mechanic to troubleshoot for me and then do the repair myself. Unfortunately, it’s hard to learn things when I don’t troubleshoot it myself.
Sometimes I’ve replaced parts without knowing for sure if the part was defective. For the past number of years I’ve been making it a point to try figuring out for sure what part is defective, before making a purchase. I have never replaced a master cylinder on my own cars and I never had a low pedal problem. This is my first time for this issue. I have replaced rotors, drums, wheel cylinders, calipers, pads, shoes, hubs and wheel bearings on vehicles I’ve owned. Making sure that my neighbor and friend is safe is of the highest priority.

I haven’t removed the brake drums because if I do, I have to pull the axles and reseal them when putting back together. I don’t want to pull a drum until I know for sure there’s a problem in there. Yesterday I used penetrating fluid and opened up the two connections at the Tee above the rear axle. I plugged two outlets of the Tee so both back wheels were removed from the system. The brake pedal behaved exactly the same way. As a reminder, the pedal sinks to the floor only with the engine running. With the engine off, the pedal is good. Also, sometimes when the engine is first started, the brake pedal kicks back against the foot.

I does that because it has hydra-boost.

I just figured out a big problem with my theory while looking at internal diagrams of master cylinder. I overlooked the fact that the master cylinder has two separate pistons and springs in it. The secondary piston could come to a stop because pressure built up in secondary line is as it should be, but if the primary line has a leak or air, the spring between the two pistons will compress further (lower pedal). Or the secondary line could have a leak or air, causing the secondary spring to compress farther and causing the primary piston and spring to follow along with it (lower pedal).

There’s one other strange thing that happens with this brake system. If I fill up both chambers to say about 1/4" from the very top, a few days later, I’ll find that not only is the secondary chamber lower a bit, but the primary chamber is increased right to the top, as though fluid is getting drawn past a seal in the secondary piston, into the space between the two pistons.