73 super brake problems

Shoes too far from the drum. Adjust all the starwheels as tight as you can (wheels won't turn). See if you have pedal.

Yes? Loosen starwheels slightly till slight drag. Re-adjust after a day or so.

No? Problem in the M/C. Seems unlikely if you got good flow of fluid at each wheel...

Speedy Jim

Did you bench bleed the master?

One thing to add here is to check the length of the MC pushrod and make sure there is some clearance between the two. If the pushrod is not adjusted properly you may never get all of the air out. I had a friend suffer through this a few years ago and it turned out his was wrong(for whatever reason) and when his MC finally bit the dust he bled the thing for days and never made any progress on it. Once we shortened the pushrod and made sure it had some clearance we bled it again and all was well.

Chris

I didn't bench bleed but followed the advise of some here and cracked the feed lines to let it bleed that way when installed. The push rod is pretty much frozen where it's been for the last 30 years so I don't think that is it. Would pressure bleeding help? Thanks again!

I swear by it.

The thing is, the new master cylinder could be a bit different length-wise than the failed unit, and if the pushrod is holding it open even slightly you will have difficulty getting it bled. It has to close all the way to work properly anyway.

It's very easy to check, just push the pedal with your hand. You should have virtually no resistance for about 1/4 to 1/2 inch and then you should feel the pushrod contact the master cylinder. If you don't have any play at all, you need to address that before bleeding again. Adjustment is pretty straightforward, there's a jamb nut you loosen, twist the pushrod to adjust and then tighten the jamb nut.

Chris

I think that you're forgetting that most of the fluid passes around the lips of the MC seals in the forward direction as you let the pedal up. There is very little vacuum in the lines, because the seals are designed and intended to pass fluid readily in this direction.

You're right, that if the pushrod is too long then the MC pistons will fall short of returning all the way and the tiny little "compensating port" in the MC will still be covered. The purpose of this port is mainly to allow fluid to flow BACK to the reservoir as the wheel cylinders relax and as the brake fluid heats up and expands under hard braking. In short, this lets the brakes release completely under all conditions.

It's true that this compensating port would also relieve any vacuum, but the lip seals in the MC don't allow much of that to exist anyway. At any rate, 99% of the fluid flow occurs around the lip seals.

The senario you propose, where the pedal goes progressively lower, does not happen. I think you might get something like this to happen if the brake fluid was readily compressible (and expandible) but this is not the case; it wouldn't be a good hydraulic fluid if it did. Its volume does not change significantly in response to a vacuum.

It's important to note that we are in complete agreement about making sure that the brake pedal pushrod has a small amount of free play.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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wheel cylinders seal can suck air, too. I've heard that happening. Odd that it's both circuits.

I've never seen that happen, but its interesting to note that the 2 would be exposed to approximately the same vacuum. The inertia and viscosity of the fluid in the lines would make the vacuum in the wheel cylinders somewhat less, but it's still interesting.

It's for this reason that I don't trust those vacuum bleeders.

I wonder if the cup seals on the wheel cylinders are made a bit stiffer for exactly this reason?

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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