Brake bleeding,et al.

[Ed Jennings]

This all started when I changed the king pins on my 62. I took the entire front brake assys off, soley to clean the backing plates. One thing led to another, and I ended up rebuilding the wheel cylinders and replacing the front linings.When I got everything back together, I tried bleeding the brakes with a Mity Vac. I had previously tried to use the Mity Vac on my 67, with no success. All I got was air and a little bit of fluid. Same on the 62. I switched back over to the little bottle with a magnet on it that I usually use, and bled the brakes without too much problem. I followed the instructions that came with the Mity Vac pretty carefully. I'm getting air from somewhere, but when I apply a vacuum before opening the bleeder, it will hold the vacuum almost indefinately. Am I getting air around the bleeder threads?

I ask about the Mity Vac because I still havent gotten quite all the air out of the system. Each time I bleed, I get a little more.The last time I got no air, but the brakes still feel a bit spongey. I assume that when operated properly the Mity Vac is supposed to do a better job of bleeding than one of the little bottle and hose rigs.

Lastly, I discovered that the right rear backing plate is loose. The outer circumference will rotate about 1/2" or more. I didn't have time or ambition tonight to pursue it, but has anyone run into this problem? The bolts that retain it look tight, so I'm wondering if something on the backing plate itself might be broken or wollowed (sp?) out. Is this a common problem? I haven't run into it before.Is it possible that motion in the backing plate is what's making my brakes feel a little spongey?

[Clem Z.]

Re: Brake bleeding,et al.

i think the air comes in around the bleeder screw threads as you are pulling a vacuum on them. a pressure bleader works the best,you can make one by fitting a schrader valve to a spare MC cap and using dry nitrogen to push the fluid thru. make sure you do not let the MC get too low. some people take a garden sprayer and put dessicant in to it and pump it up to do the same thing. we asphalt circle track guys always have dry nitrogen around for filling out race tires to keep out the moisture out so the tires do not build up pressure and changing the stagger.

[Clem Z.]

PS

speed bleeders from the race car shops help also. these bleeders have a check valve built in so all you have to do loosen them and you never have to worry about air going back into them even if you use the old pump the pedal and hold deal. just pump and you do not have to close them till you have all the air out.

[Ed Jennings]

Re: PS

That's basically what the little bottle and hose does. Creats a column of fluid above the valve and gravity keeps air from getting back in.

[Kenneth S.]

Re: PS

Had the same problem as you with the MightyVac. I too think the air is leaking around the bleeder valve. I thought of putting something like pipe dope around the threads but don't want to get that into the brake fluid. Maybe someone has a solution as primary use of the MightyVac is supposed to be bleeding brakes.

[Ed Jennings]

Re: PS

I bought mine to test the cranking siognal valve on my fuel injection. Works great for that. Several people on this board swear by the Mity Vac for brake bleeding. There's something I'm not doing right.

[Duke W.]

Re: PS

I agree that it might not be a good idea to use pipe dope or any other sealant on the bleeder valve threads as it may not be compatible with glycol brake fluid, but I think teflon tape is okay.

Duke

[John H.]

Re: PS

I've used the MityVac for many years on numerous cars with good results - occasionally I find that the little black plastic adapter that snaps over the end of the bleeder screw doesn't fit well, and I just remove the adapter from the vinyl tube and stick the tube itself over the end of the bleeder to get a good seal. The "Speed-Bleeders" have a sealer substance on the threads to prevent sucking air past the bleeder threads when you take your foot off the pedal; I haven't used them, but they sound like a great idea.

[Craig S.]

Re: PS

Another great alternative is Phoenix Systems, here in Tucson. Their systems allow pressure bleed, vacuum bleed, and RFI bleeding (pressure from the wheel/caliper back to the master cyl. I have their maxi-ject pro and it's a nice unit...Craig

http://www.phxsyss.com/

[Bill Clippinger]

Re: Brake bleeding,et al.

Another thing to consider. Although I've never used a vacuum-type brake bleeder, let me quote you this section from a book on brake systems.

"Vaccum bleeding sounds neat, but it has a potential problem. Brake cylinder cup-type seals are meant to work with fluid pressure greater than the outside air pressure. With the vacuum pump, fluid pressure is below the outside air (pressure). This may draw in air past the seals, unless your system has cup expanders."

So perhaps that might be the problem.

[John H.]

Vacuum Bleeding And Seals

Good point, especially for older cars. However, all domestically-produced GM cars since 1966-67 have seals designed specifically for extreme high-vacuum bleeding. In production, the bleeders were never touched. After the entire brake system was assembled on the chassis (before body drop) including the master cylinder (booster was on the body when power brakes were specified), an adapter device was clamped to the master cylinder which sealed it, and an internal probe sealed to the small compensating port in the bottom of the reservoir. The adapter was connected to an "Evacuate-and-Fill" machine which first drew an extremely high vacuum throughout the system (and monitored the rate of vacuum increase to check for gross leaks while doing so); once the required vacuum was drawn, the machine injected brake fluid at 50 psi into the system through the same port, and bleed/fill was complete. In the next station, another adapter device was attached to the master cylinder which clamped it solidly by the attaching ears; a hydraulically-actuated plunger then applied pressure to the piston at the rear of the master cylinder, bringing the brake fluid pressure in the system to 1600-1800 psi, and after the system stabilized (to compensate for hose expansion) monitored the applied hydraulic pressure looking for any decay, which would indicate a leak, and the length of stroke applied indicated any pedal softness, or air in the system. When this test was complete and passed within limits, the brake system was certified leak-free. I designed this system and supervised its installation in the first few plants that received it.

Final certification of the brake system was done in the roll-test machine, which had load cells at each wheel's rollers that measured dynamic braking effort at each wheel during the roll-test cycle. You could conceivably have a fitting somewhere in the system that wasn't drilled all the way through, and that wouldn't show a leak at the chassis line evacuate/fill/test sequence, but would give you only two- or three-wheel brakes on the finished car.

The only time the bleeders were ever touched was if a chassis failed the evacuate/fill/test sequence and needed a repair "re-bleed", which was done with a pressure bleeder ball; the evacuate/fill equipment only worked on a "dry" system.