Drum brake bleeding

[Ed Jennings]

I recently did some brake work on the front of my 62. Basically, all I did was rebuild the wheel cylinders and replace the shoes. The later was sort of an afterthought, as the existing shoes were only partially worn. The wheel cylinders were not leaking, but even though I had flushed the system the cylinders were full of "crud".
The problem is that after bleeding the brakes I still can "pump up" the pedal. Every time I go back to bleed again, I get a bit of air from the right front cylinder. There are NO external signs of leakage, period. I am familiar with the phenomen of disc brakes "sucking air", but have never had any experience like this with drum brakes. Is there anywhere that I can be getting air back in the system that isn't going to leak fluid? What about the bleeder valves themselves? They are probably the originals.The car still has what are likely the original 1-3/16" wheel cylinders.
Incidently, I have bled all 4 wheels, with and without an assistant, using various methods, including the "assistant pump 'em up and hold 'em while I open the bleeder". Having run close to a quart of fluid through the system I think I have gotten all the air out at least once and that air is getting back from somewhere. Any suggestions would be much appreciated.

[Duke W.]

Re: Drum brake bleeding

A leak in the m/c can pull in air and cause a spongy pedal that has to be pumped up. In this case you would be able to bleed the air from any of the wheel cylinders. If you are certain that air can only be bled from the one wheel cylinder, then it is suspect, and I'd recommend you remove it for disassembly and inspection. Even if nothing is obviously wrong, put it together with a new wheel cylinder repair kit.

I doubt if the bleeder valve is the problem. If they don't leak under hydraulic pressure from the applied brake pedal, then they are likely not the culprit, but some pitting or a bad cup in a wheel cylinder could draw in air when they retract, and I think it's possible for the leak to be one way i.e. they can pull in air on retraction, but don't leak fluid under hydraulic pressure.

That debris you see in the fluid comes from three sources IMO - rust particles from internal corrosion, mold release compound from the rubber parts, and bits of rubber from the cups that is torn off from sliding friction. Eventually this debris can plug the compensating port in the m/c and result in loss of brake function. Periodic flushing both removes absorbed moisture and tends to flush out the debris that accumulates over time.

Duke

[Ed Jennings]

Re: Drum brake bleeding

Since posting earlier, I have discovered that indeed air can be found in both front cylinders. I had removed, cleaned, and honed both cylinders and installed new kits. Sounds like maybe I didn't do a very good job. Guess I'll have to lick my cat over.

[Duke W.]

Check the rears, too

If you're getting air from all four corners, I would suspect the m/c. It could be pulling in air when the piston retracts. A plugged compensation port from debris could be the culprit here. If there is a lot of debris in the m/c reservoir, clean it out and make sure the compensation port is clear, then give the system a good flushing. If that's not it, it might be time to O/H the m/c.

Duke

[Ed Jennings]

Re: Check the rears, too

Will do. I rebuilt the front cylinders because the 1-3/16 cylinders are no longer available. I can replace with 1-1/8", which is correct for years up through 60 and for 61-62 with HD brakes. Other than the originality issue, is there any good reason to not replace with the 1-1/8"? Howcome the 61-62 is different to start with?

[Duke W.]

Re: Check the rears, too

Smaller front wheel cylinders will result in more rear brake bias. The hydraulic bias is equal to the ratio of front and rear wheel cylinder areas to total wheel cylider area.

I expect that GM increased the size of the front wheel cylinders to increase front bias and reduce the incidence ofrear wheel lockup . GM has always seemed to overbias the rear brakes. The early eighties X-body (Chevy Citation) was, perhaps, the worst example.

Tire adhesion has a lot to do with bias selection. A sticky tire will require more front bias because it will generate more weight transfer to the front under heavy braking. I want the front to lock just before the rears. If the front locks the car tends to go straight and some pedal modulation will usually unlock the front wheel(s). When the rears lock, directional stability is lost - the car wants to spin - and because the rear is so lightly loaded at high deceleration, you literally have to get off the brakes completely to get a rear unlocked.

Duke

[Ed Jennings]

Re: Check the rears, too

I had an 82 Z28 that would swap ends in a heartbeat if you locked the brakes. Otherwise an incredibly good handling car at the time. I'll keep working with the 1-3/16" cylinders.

[Duke W.]

Re: Check the rears, too

My Cosworth Vega had a rear lockup problem, too. In the wet it was downright unsafe. In preparation for race track hot lapping I replaced the solid Vega rotors with the vented Monza rotor and their companion larger piston calibration which reduced rear bias substantially. All of a sudden I could outbrake Porshes and take them on the inside going into Turn 7 at Riverside.

When the SWC is done I suspect I'll be looking for some smaller rear wheel cylinders.

I think the reason why GM overbiases the rear is that they set up the brake bias for "design load" which would be a full load of passengers and cargo. Since most sedans are rarely loaded to capacity, the end result is overbiased rear brakes. It's not as bad a problem on a Corvette because of no capacity for rear passengers.

Duke