1992 Cadillac Deville

SEAN03

MEMBER
1992 CADILLAC DEVILLE
135,000 MILES

My neighbor loaned his brake bleeder. It is a mighty vac 6820. I tried to use it and it wont work. I put a hose on the pump to the can top then I put brake fluid in the cup then I took another hose and put it on the other side of the can to the right rear wheel cylinder screw. I opened the bleeder up and started to pull the handle on the vacuum pump and all it did was pull the fluid out but their was gaps it was fluid no fluid and it was coming out as air basically. I read the book and it said to put a coupler in it that downsized it to 5/16 I think. The problem was the 5/16 is too small of a hole to put it on my bleeder screw. Any idea what else I can do. My last resorte is to have someone pump the brakes then I will open the bleeder valve and have them push it down to the floor and hold it then I will close it and have them release it. I will do it until it comes out as liquid. If you can think of a way to get that mighty vac to work let me know. Thanks.

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

First of all, you never never ever push the brake pedal to the floor unless it's a new master cylinder. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. Running the pedal to the floor runs the lips seals over that crud and can rip them leading to a slowly sinking pedal.

If you're using a vacuum bleeder, I've always run into air coming out that is actually sneaking in alongside the threads of the bleeder screw. As long as dark brake fluid comes out at times, you know you're pulling new fluid down from the master cylinder. By the time the fluid turns clear, you should have pulled enough fluid to get the air out of that line unless you let the reservoir run dry.

Next, I'm pretty sure you have four steel lines coming out of the master cylinder. Most front-wheel-drive vehicles use a split-diagonal hydraulic system so the right rear and left front are on the same circuit. GM has a valve in the master cylinder that will trip and block two ports when there's enough of a pressure difference between the two circuits, as in a leak or when pushing the brake pedal to work the front caliper pistons out after replacing the pads. If you never push the pedal more than about half-way down you won't trip that valve, but if you do, when one piston gets out far enough for the pads to contact the rotor, pressure will start to build in that hydraulic circuit but not the other one. The valve will trip and no fluid will go to two wheels. And you won't be able to bleed them.

The only way I have ever found to reseat that valve is to go to one of the wheels that won't flow any fluid, open the bleeder screw, and give it a very short, quick burst of compressed air, then let it gravity bleed.