Master Cylinder Differences

[sholsten]

I have a 1990 GMC /w 6.2L. The master cylinder is bypassing and going almost to the floor like I've had others to do.

Is there any differences in one for a gas burner and our Diesels? Thanks in advance for replies...

[More Power]

When I converted Lil Red's 1989 vacuum assisted braking system to a 1989 hydraulic assist, I needed to buy a new master cylinder to fit onto the hydraulic brake booster. The bores for the internal stepped alignment flange are of a different diameter. The vac master uses a slightly larger bore. I thought about machining the flange, but in the end, just bought a new one. Didn't want to take a chance on such an important piece.

As a side note, try unplugging the electrical connector to the RWAL (Rear Wheel Anti-Lock) module located beneath the master cylinder, to see whether a defective module/wiring/sensor is causing the hydraulic pressure dump. When working normally, it'll dump pressure when the vehicle is stopped (to prevent the rear wheels from locking - like while driving on slick roads), but not when the VSS (vehicle speed sensor) indicates a moving vehicle.

Jim

[sholsten]

Thanks Jim. You just saved me a headache thinking about trying one for a gasser. I've had the module under the master unplugged for about 4 years because it was blowing fuses.

[sholsten]

I got out today and started checking things out. The master cylinder is losing quite a bit of brake fluid. I think when my son was driving it that he let it get too low and got a bunch of air because they'll pump up tight and then lose it. The wheels all appear to be dry, but where could the brake fluid be going? I know the gasser's vacuum boosters can suck the fluid in the engine if the master cylinder is by-passing. Can the brake fluid get into the hydro-boost if the master cylinder is by-passing? I'm stumped....

[DmaxMaverick]

The fluid is leaking out somewhere. It won't leak into the HB unit. Perhaps dripping onto the exhaust manifold?

If it bleeds down while standing on the pedal, the MC is shot, or you have an external leak. If it's just air, it will be spongy, but won't bleed down once the pedal is hard.

[sholsten]

Thanks for the response. The pedal stays hard when I get it pumped up and it'll stop on a dime. Jim mentioned above that the RWAB could be screwing up. I've had the module unplugged for about 4 yrs without any problems. I wish there was a way to bypass that with the lines, but I don't see how. :confused:

[DmaxMaverick]

Still, if it's losing fluid, it's getting out somewhere. If your ABS module is electrically disabled, it should have no effect on brake operation. It could leak, but you should be able to see it somewhere. You could bypass it with flared steel lines pretty easily (requires some hydraulic tubing experience). I suggest finding the leak before anything else.

[sholsten]

Thanks again. I thought the lines would be a pain to try to bend & etc without kinking or breaking them. I'm going to have my son to crawl under it in the next couple of days when this rain moves out and look close at all four wheels for the leak. The MC and associated valves are dry all around them.

[DmaxMaverick]

If you do a DIY on the bypass, be sure to use the correct tools. You'll need the tubing, hardware (tube nuts, sleeves, and couplers), the correct size tubing bender, small tubing cutter, and a flaring tool (all relatively inexpensive). If you have it done, it can cost quite a bit. If you have little or no experience with this, get enough tubing to practice bending and flaring (the bent tubing will be the only discarded part). It isn't hard, and won't take much time to be proficient if you are handy with other mechanical stuff and tools. PM or email me if you need a step by step. I have extensive experience with this.

An alternative is to leave the module in place if it isn't leaking. If it isn't leaking now, it may never. The easiest repair you'll ever make is the one you don't have to.

[sholsten]

We finally found the problem. The right rear wheel cylinder is leaking. It's going in the shop tomorrow. :(