1994 GMC C1500 brake bleeding

CARHOP

MEMBER
1994 GMC C1500
5.0L
V8
2WD
AUTOMATIC
139,000 MILES

Some brake bleeding instructions for my truck specify using a proportioning valve depressor tool when bleeding rear brakes. My proportioning valve has a pin in front of it covered by a rubber cap. I can push it in about 1/8" without the tool, but when I do it drips brake fluid. What is the purpose of this pin? Does the valve depressor tool also prevent leakage of brake fluid? Thank you.

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

Forget that tool. I've read those directions too but never had to resort to opening that valve. That's the "metering" valve and it's for the front disc brakes. It is also called the "hold-off" valve. It's purpose is to prevent brake fluid from flowing to the disc brake calipers until pressure reaches around 15 psi. That is to give the rear shoes time to expand and make contact with the drums. That way all four brakes begin to apply evenly and at the same time.

If you need to bleed the brake hydraulic system, just open all the bleeder screws and let them gravity-bleed. Loosen the cover on the reservoir to prevent vacuum from impeding the free flow of brake fluid. When fluid starts flowing from one of them, close it and wait for the next one. When all four bleeders are closed, "irritate" the brake pedal a little by hand, then open each bleeder once more for a couple of seconds to remove the air bubbles that washed into the wheel cylinders and calipers.

Some people think you have to pedal-bleed with a helper, but very few professionals do that unless they're in a hurry. If you do that, never, never, ever push the brake pedal more than halfway to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel in the master cylinder. Pushing the brake pedal over halfway down runs the pistons over that crud and can rip the lip seals. That will cause a slowly-sinking brake pedal that may not show up for a few days. This doesn't apply to a master cylinder that's less than about a year old.

If you're replacing the master cylinder, there's an easy trick to avoid having to bleed at the wheels or even remove them.

When adding brake fluid, be sure to not get any hint of petroleum product in there with it. That includes engine oil, transmission fluid, power steering fluid, and axle grease. Professionals even wash their hands when working with brake parts to prevent getting fingerprint grease in the brake fluid. Also, don't use penetrating oil to get brake parts apart if they contact brake fluid.