BC

Since that post I have been outside and pushed fluid up into reservoir from all corners. Then atbthat corner pulled vacuum at the brake port.

Absolutley no change. I can’t believe this.

dr bob

Brendan --

When you did the "bench bleed", did you get full flow from all ports coming back into the reservoir? That would be an indication that the master cylinder is working. The bench-bleed offers minimal back pressure, but would at least be a start. Are you getting good flow through the open bleeder when you are pedal-bleeding from MC to caliper? Are the pads staying close to the rotors after a few pedal pushes? It seems unlikely that both circuits are bypassing internally in the master cylinder, but it could happen.

BC

Yes full flow during bench bleed. I bled both the previous late two port master as well as this early 4 port master (one port for each circuit is bolted off as I’m not using them).

Great flow from brake calipers (front S and rear early) during bleed. Can see fluid flowing at all times. Entire system flows new fluid with no bubble any time fluid flows. I have seen more bubbles on used but working systems at bleed time.

Some interesting observations today:

I can hand turn front wheels until partner presses brake pedal 4 inches towards floor. Only them is it getting harder.

I now have an s4 booster and an early master. Am I having a mismatch issue? Is the stroke not enough, like on a clutch master for the middle cars?

I will possibly remove wilwood prop valve from rear circuit tomrrow to see if it is somehow affecting this but if the circuits are separate in this early master, that doesn’t make sense.

BC

Same soft brake pedal between two different bench bled masters. I am missing something.

hacker-pschorr

After talking to BC a short while ago, I agree there is something going on here with hardware. Almost like the master isn't reaching full travel or something.

How much adjust-ability is there in the brake pedal? For a test move it out (towards the driver) as far as you can in the "rest" position and adjust the rod accordingly. This way you'll get more travel when pushing in????

I dunno, just thinking out loud.

The Forgotten On

I have a used S4 booster on my 81 with its stock 4 port master and it works well with 993 calipers which have larger pistons than the S4 equivalent. The pedal is rock solid when the booster has no vacuum.

I personally think it is the aftermarket proportioning valve as I have had trouble with aftermarket parts and it just seems like the main variable in your system.

Test the system without it to see if there is a difference to remove another variable from your diagnosing.

Your system to me seems laid out similarly to the system Porsche used after 83 without abs, so you can check some documents about it in the WSM to see if any of that info can help you.

BC

Tomorrow I’ll play around with the rod under the dash area and see what I can change.

I will also be taking out the proportion valve to see if there is any difference. It’s a possibility only because it is a variable - as the front brakes are just as bad as the rears and it’s two separate circuits. Or supposed to be.

BC

Blake - did you leave in the two prop valves below the master in your car?

The Forgotten On

I did. I just took them out to repaint them, otherwise they the are original 33 bar bias valves.

dr bob

With the mixed boosters and master cylinders, let's go back to some basics. At the pedal, can you feel a small amount of free movement before the pedal pushrod-and-booster make contact with the piston in the master cylinder? There's obviously "some" as you are able to pedal bleed the system, but that really needs to be a minimum amount. I'm sure there's a WSM value for this, but "just barely" is good, much more can be too much.

BC

A small amount of free movement is there but not much. What I have done is to move in the bracket at the end of the booster arm to make sure I am allowing full retraction.

BC

Removed the bias valve so all plumbing is just lines and junctions from master to calipers. These are early s single piston calipers.

No change after bleeding a bit. Drove car, and it doesn’t really stop at all. Parking brake stops car much quicker than brakes.

Next is to remove calipers I guess. I’m just changing parts at this point with no real idea.

Imo000

Clamp the flex lines at the calipers and see if the pedal firms up or not.

BC

Front I can, rears are new ss Teflon lines.

hacker-pschorr

Then use vice grips.

EDIT - this is a joke, nobody finding this thread in the future should ever do this. But if you do, post photos.