This is on a '79 RX7 road race car (not sure what model the booster and master are from). No proportioning valve, but has a brake bias valve. Brakes worked great for last season and a half

This is a long story, but details may be important

2 weeks ago I pushed the brakes with the right front rotor off. After that the system would not bleed. Checked the that caliper and it seemed to be weeping fluid around the seal. Got a caliper off the parts car. Damaged the fitting on the steel braided braked line, replace with used steel braided line. Brakes still would not get a firm as usual. Had a race weekend and the brakes would go from spongy to almost no brakes in 5-10 mins. If I pumped first I could get enough pedal pressure to get throught the race.
Pedal developed a thump at the bottom of travel when car was off while bleeding the brakes between each track session. Used almost 4 500ml cans of fluid over two days of bleeding. No noticable leaks.

This week I Bought rebuilt master that looked the same as unit on car (two bolts into booster). Bleed master with bleed screw while on car, did not bench bleed. Bleed all 4 wheels starting furthest away. Still a bit spongy, and still had a thump(s) in the pedal travel. Had booster rebuilt. re-bled as above, pedal is worse than before and the thump is still there.

Also of note, at the start of the year when I changed pads in the rear, I used a "c" clamp to press the piston. I did not "screw" them in as I have been told is the correct procedure. I raced 4 weekends with out brake touble.

I race this weekend. It is the last race of the year and I am leading in points so I need to get this fix. What are your suggestions?

Thanks

 

hmmm... congrats on leading the points

 

"Capiler from parts car", are you sure it's any good? If it sat for any length of time, it may be rusty inside. Might be time to invest in a reman caliper, or a rebuild kit for the one you removed, since you blew the seals in it.
As for the rear calipers, well you destroyed the park brake actuators, at a minimum.

 

If you did not bench bleed the Master, then there is quite possibly air up beyond the ports where the fluid is sent to each wheel. You will never get good hydraulic action with air in the master.

The easy trick may be to jack the rear of the car way up so the master is at an angle upward at the booster. That may allow the air bubble to perculate back and consequently get flushed out the lines. But I fear you will need to remove the master, bench bleed it and reinstall, and re bleed the lines.


Another possibility is that is you are using Speed Bleeders to bleed, that they are leaking around the threads of the bleed fitting. I've seen that a lot. It allows air to get sucked back in as you release the pedal.

Good luck racin, but get that spongy feel gone first. Can't race without confidence in your brakes!