[marlene kit car]

For my Marlene kit car I am using a 1988 Mark VII rear end with 11 inch disks and calipers just like it came from the factory. Front are 11 inch Granda disks with Chevy metric calipers (standard Granda coversion kit), 1976 Mustang II master cyclinder and booster. I have great solid brakes with 4 inches of pedal floor clearance with the motor off! When I start the motor and the booster kicks in all brake disappears?? The pedal doesn't hit the floor (about 3/4" of clearance) but it feels like the master cyclinder bottoms out?? When I shut the engine off everything seems and feels like before, but with the engine running brakes are gone?? I check everything 10 times and master cyclinder is always full (its a rebuilt), pedal always fine when engine not running, etc.
Thanks for your help.
pushing 70 Ralph

ps: I tried 6 times to correct Granada and cylinder, to no avail.

[pepi]

fat finger

[pepi]

what about the booster is it new or used, without power to the brakes (vacuum) all should work normal. The purpose of the booster is assistance.

When I shut the engine off everything seems and feels like before, but with the engine running brakes are gone?? I check everything 10 times and master cylinder is always full (its a rebuilt), pedal always fine when engine not running, etc.

Do you have a proportional valve plumbed? Are you saying that the brakes are gone as if unable to stop or are you thinking because the pedal drops you have no brakes?

[marlene kit car]

I have strong, working brakes when the motor is off or the vacuum line is disconnected. However, when the engine is running or the vacuum line is hooked up the pedal goes all the way to the floor (about 3/4 inch off the floor). It feels like the master cylinder is bottoming out?? It is a rebuilt 1976 Mustang II booster with a 1976 rebuilt master cylinder. I have continually checked fluid level in the master cylinder and all is well. It is weird because all the booster should do is help. I have the original 1976 Mustang II (the donor car) proportioning valve. Its like the booster takes away all the pedal to master cylinder pressure??
Thanks for your help,
Ralph

[Henry Highrise]

Sounds like your re-built booster is bad...that is exactly how one acts when the diaphram is bad. I installed three re-built boosters on a truck one time before I got a good one. Thats the way it goes sometimes.

[marlene kit car]

OK, today I jacked up the rear end and started the motor and put it in gear. I have brakes, but they are down near the floor. I disconnected the booster vacuum line as part of the testing. Every thing seemed to be the same and the brakes were still down at the floor when they began to engage. However, when I reconnected the vacuum line the pedal was up about 3/4 inch more than before?? The brakes stopped the rear wheels easily in forward and reverse with or without the booster, I just don't know where all my pedal travel went to?? I got to thinking maybe the power booster was actuating the master cylinder somehow, but the rear wheels turned freely in and out of gear. I guess I just need to have the pedal higher off of the floor than I wanted. Right now it is 5 inches off the floor and 4 inches with my foot hard on the pedal without the motor running, but as soon as I start the motor all firmness disappears and all pedal travel, too. Are there other small boosters that don't need so much travel to work or can I adjust the booster push rod out more? I have it to where when you put the master cylinder on it holds the master cylinder out about 1/16 of an inch before you tighten up the nuts. I couldn't find any instructions on this aspect of the booster/master cylinder. The booster push rod has an adjustable tip on the master cylinder side.
Thanks a million for your help,
Ralph

[home brew]

Did you nto read Henry's answer?

Originally Posted by Henry Highrise

Sounds like your re-built booster is bad...that is exactly how one acts when the diaphram is bad. I installed three re-built boosters on a truck one time before I got a good one. Thats the way it goes sometimes.

[DrChop]

Clearance between the booster rod and the master cylinder piston should be zero. DO NOT dial in more on the adjustment. Doing this allows the booster, at rest, to apply the piston, which covers the takeup port in the master. As the pads wear the pedal will drop lower and lower, with no replacement fluid allowed by the blocked port.

Change the booster out, adjust the pushrod for zero to a fraction of an inch of play, and rebleed the system with a helper, starting at the furthest wheel from the master and working until only clean, non-aerated fluid comes out the bleeder, before moving to the next-furthest wheel.

When you have all 4 wheels bled your pedal should be good. If you still have excess travel, check and adjust the rear drum adjusters for a slight amount of drag, as too much slop here translates to a low pedal. In your case you are running rear discs, so this part does'nt apply to this problem, but I include it because it may help someone else out...


Also keep in mind you can gently clamp off the flex hoses with locking pliers to check if it is a fluid problem, or to isolate the low pedal to a specific part of the system...

Right now I'm agreeing with Henry, that the booster is bad. However, a working booster will make the pedal feel softer/lower than it is with no vacuum applied/engine off.

Doc

[marlene kit car]

Here's what I did. I called Cardone and talked to one of their techs. At his suggestion I removed both brake lines from the master cylinder and put in metal plugs. Then I fired up the engine and tested the master cylinder (rebuilt by Cardone). For a moment it was like a rock. Then down to the floor it went! I tried it a couple of times with the same result. Then with the motor still running I started bleeding the master cylinder. No luck! No matter if I opened the front or the rear plug the rear reservoir went down and the front kept getting fuller until it ran over. I kept the rear reservoir full. I assume the master cylinder was bad. This is the second bad one I got. The booster worked like a dream. Without the motor running I could stand on the brakes with all my force and they wouldn't go down, but introduce the booster and down they went!
Thanks for all your help and suggestions!
I hope this experience helps someone else. Be careful of the rebuilt. Most are good some are not. Last Friday I started to buy a rebuilt water pump, but notice the seal was damaged so I got a new one for $15.00 more.
PS. The Cardone tech was extremely helpful and went a couple of extra miles to help me.