Help with brakes

[R5335]

Mark, I hadn't seen anything on this since you last wrote on 11/10/24. So I too have just run into this issue. I'm hoping you found an answer.

My story is similar. The original dual master cylinder was leaking and some fluid was getting into the booster. So I replaced the master cylinder. However, my mistake, I extended the booster push rod to what the manual said to 1.25 inches. Didn't take long to burn up the calipers. So replaced the calipers, had the rotors turned. Reset the push rod from the booster to match the new master cylinder, in my case 1" (actually 31/32). Then off I went and for a few rides ok. Shortly thereafter, pedal was very hard to push and brake the car. OK, videos and discussions with folks says it's a bad booster. So, sent that off to have it rebuilt. Report back was yes brake fluid got in and diaphragm was cracked and the wrong check valve. So, finally got that back. Put it all together, bench bled master cylinder, bleed the system 3 times. Now when the engine is off, brakes are hard to push as would be expected. However, like you, now when I turn the engine on, brake pedal goes to the floor! On top of that, the engine stumbles, loses some vacuum, about 5" Hg. Bleed brakes again, no change. Talking to the fellow who rebuilt the booster says it's a bad master cylinder. Damn thing is only a few hours old. I'm using DOT 3 fluid if that means anything. I'm at a loss as to what the issue is hence why I read your forum with great interest. It also appears when doing a search on the AOIA and SDC forums, this has been a familiar problem with a fair number of people. 

[Mark L]

No, sorry, nothing new to report. I haven't done any work on my Avanti since last fall. It's still sitting on the driveway on jack stands under a cover. I have a garage project I must finish before June 2. Once that's done, I can get back to work on the Avanti.

One thing I did find at the SDC site is that someone found the rear disk brake caliper brackets that Jim designed hold the calipers in auch a way that the bleed valves are pointed down slightly, which might be creating a pocket inside the cylinder that's holding air away from the valves. When I get back at it, I'm planning to remove at least one of the caliper bolts so I can pivot the caliper so that the bleed valve is at top dead center.

I also bought a new master cylinder from a local source. I think I may have damaged the seals inside one of the previous units when I experimented with the port to the rear brakes capped off. I would have attempted to rebuild it, but Raybestos does not sell replacement parts for it.

Edited May 20 by Mark L

[Mark L]

Today I was finally able to get back to working on the brakes. This evening I bench bled the new master cylinder I mentioned in my May 19 post above. When I took it out of the box, I was pleasantly surprised that it's actually an aluminum master cylinder, and it does not have a hole pre-drilled and threaded for the brake light switch. That's just fine, because I removed the hydraulic brake light switch from the circuit and I'm installimg a dedicated mechanical switch at the brake pedal arm.

I uncovered and aired out the car a few weeks ago. I was surprised how rusty the new cast iron master cylinder had become since last fall. The aluminum MC will probably stay looking nice for a longer time.

Over the next few days I'll get the MC installed and start bleeding the system for the umpteenth time. When I get to the rear calipers, I'm going to remove at least one of the bolts and make sure the bleed valve is not pointing down as it is now with the bolts installed.