Just finished installing my 4-wheel disc & b-body spindle swap and I am having trouble with pedal feel.
It was originally a 4wheel power drum car and I am using the original distribution block on the frame, the car is a 1970, everything else is new (or re-manufactured) asides from several brake hard lines. Replaced all calipers, master cylinder, booster, adjustable proportioning valve ,,, everything. I bench bleed the master and all the lines for hours. I have not yet road tested the brakes as I have not yet got the car aligned even in the ballpark, but just from tests in the driveway I know that all calipers are at least working and can hold the car still. Problem is that when the car is off, pedal feel is good, but as soon as I start the car pedal goes straight to the floor and the brake light comes on. I'm pretty sure that I bleed everything correctly. Any suggestions??? how do I know if I have a bad master??? and if it is a bad master wouldn't the pedal feel be the same with the car running or not
I can list all the components upon request but I am hoping that this symptom is more common and some general advice will solve it.

Thanks in advance guys. Getting anxious to get on the road...

Sounds like a problem with the power booster if you're still using it. That would be my guess. Since it only occurs when you start the car and that is the only fuction of the braking system that is affected by the car being running or not.

thought about that but the power booster is new (well - remanufactured), its an 8" dual diaphram unit originally used on second generation trans-ams with four wheel disc brakes and another memeber on this board uses one on a disc/drum car. I thought that possibly I just didn't have enough effort in my foot to realize a weak pedal without the vacume assist... how do you know if a booster is bad? or could it be pulling too much for my application?

Been doing some reading on the MP Brakes website troubleshooting section - well put together.
Came across this:

"Wrong calipers
If you are using a low drag or metric GM caliper without the correct quick take up master cylinder you will never get a pedal. In 1982 GM started using the smaller metric caliper because it's design retracted the caliper piston up to .125" ( a regular caliper retracts 2-4 thousandth's of an inch). This is a lot and without the correct quick take up master to move the caliper piston sufficiently your brake pedal will be extremely spongy. The metric low drag caliper can be identified by a chamfered leading edge of the square cut groove for the piston seal. "

My rear calipers are '82-88 Camaro front calipers and the fittings are indeed metric, but are they the dreaded low drag style mentioned in the above insert??? Do I need a quick take-up master to match these calipers? graemlins/clonk.gif I know that others on this site are using the same, anyone have any insight??? It would be great to start eliminating some possible factors. :confused:

I had the same problem w/ my chevelle w/ front discs. Ended up being the power booster. Sounds like yours could be a vacuum problem not enough?

It's probably not related to your problem, but make sure you plumb the adjustable proportioning valve AFTER the distribution / safety switch block. If you install it before, there will be a pressure differential across the front and rear ports on the block, which will trip the switch when you get on the brakes. The pedal will drop a little when this happens as more fluid is needed to fill up the "void" when the switch moves.

Troy