jasonthatsme

Thanks in advance for any help you can give. I have a 1992 G20 Sportvan conversion, 5.7L. Van is in great shape for being 20 yrs old. It had a slight pull to the left so I replaced the pads rotors and calipers and the shoes and drums on the back. Needed it anyway. When I was bleeding the system I noticed the fluid came out slower on the left side, which I assume was causing the pull. The pull continued after replacing everything so I figured the problem was in the lines. Well, a couple days ago I completely lost all pedal stiffness and it is really hard to stop. Brake warning light is on. I'm thinking bad master cylinder but I guess it could be the vacuum booster too. What do you guys think? Any way to test?

canucklehead

Do you have a blown line? The barke light will come on if pressure is lost on one side of the system, and the valve in the master cylinder moves over to block off the low pressure side. You may only have one front and one rear working now.

Did you notice any increased wear on the inner pad than the outer. If so, that could cause a pull. My caliper slide pins were corroded and needed to be wire brushed and lubed to make the caliper float evenly.

Did you not do the rear wheel cylinders too? Might have a seized one.

To test the vaccum booster. You sit in the drivers seat, engine off. Press the brake pedal several times to bleed out the vaccum in the booster and finish by holding the pedal down with your foot. Keep your foot on the brake and start the engine. Your foot should go further toward the floor once the engine is running. If it does not, then your booster is faulty, or you have a vaccum leak.

jasonthatsme

I don't think there is a blown line. They do look ok other than a little old. Also, there is no fluid leakage that I can see...Nothing on the ground and reservoir level is the same. I'd assume there would be fluid leakage with a blown line. As far as the caliper bolts, everything is brand new...pads, rotors, and calipers and was well lubed when installed. Did your steps to test vacuum booster and that checks out. I'm probably going with the master cylinder is faulty. The only thing that was confusing me was the difference in fluid pressure on the right and left sides. Can the master cylinder cause that or is there something else in the system that regulates right and left side pressure?

canucklehead

The system should be diagonal. So if you have a problem on one side of the system is should be on one front and the opposite rear.
Maybe that float in the master c is over to one side. Try clamping the bad side front caliper and pushing some fluid back towards the master and see if it fixes the brake light being on.

200OZ

Sounds like you have 2 different problems. First off, the older G vans have a front/back split bake system, not diagonal. The master cylinder will not cause unequal brake pressure left to right, it's all one circuit. The combination valve splits the system left to right. I'd be willing to bet (assuming there are no leaks, and the system is bled properly) that you may have a bad flex hose on one side, you should replace both anyway.
When bleeding brakes by pumping the brake pedal on older vehicles, you usually end up pushing the master cylinder piston seals past there normal travel limits. What lays in those untraveled areas? Dirt and corrosion, and the seals just past through all that junk, and you may have compromised the seals ability to seal and produce pressure.

If you're still having trouble figuring out what wheel(s) are not braking properly, try and do some quick short stops on a pebbled road or parking lot. Take a look at the skid marks, the shorter one(s) may have less brake pressure. This isn't a sure test, but it may help, it's worked for me before. Let us know what you find.

Mike

tech2

if the red brake warning light is on, check the brake fluid level. if its low, there is a leak...maybe your 20 year old brake lines have burst. Do you live in the rust belt.

all fWD vehicle have diagonal split braking systems. All RWD vehicles are split front to back.
combination valves do not split brake systems left to right.

200OZ

The OP said he checked the lines for leaks, and the reservoir is at the same level. Most older GM cars and trucks (ie. 1992 G van), the comb. valve split the left and right front brake lines.

tech2

combinations valve house the differential pressure switch proportioning vavles and sometimes metering valves if the vehicle has rear drums. Setting these systems up for a left/right split on a van that has a front/rear master cylinder division makes absolutely no sense.

200OZ

Maybe I didn't explain myself very well and we aren't on the same page. Gm used the same combination valve for 20 some years ending in the early 90's. It has 2 ports for the front brakes (L & R), and one port for the rear brakes, I'm reasonably sure the '92 G vans still used the same old combination valve with the 2 front brake ports. I could be wrong, and if I am, sorry for the confusion. Here's a cut-a-way pic of a old GM Combination valve.
Even a 4 wheel disc car/truck will use a metering valve, rear brakes always use less pressure than the front brakes.

tech2

metering valves are not needed on a 4 disc brake system. their sole purpose is to delay the front calipers from applying until the rear drum return spring tension is overcome. you are confusing a proportioning valve with a metering valve.