I installed a kit to convert my manual disc/drum brakes to power disc/drum. The kit came with the master cylinder, booster, adjutable proportioning valve, lines, etc. I bench bled the MC, installed all of the parts, put the prop valve after the distribution block as the instructions stated, and the bled the system at all 4 wheels. The problem is that the brake pedal goes all the way down when pushed and the brake warning light on the dash lights up. It's like there's no pressure. It will build and hold pressure if I pump the pedal and hold it but when I let off the pedal and push it again there's none. I have bled the system 3 times, adjusted the rear brakes, checked over and over for leaks and found none. The master cylinder is not losing any fluid. I am at a loss for what to do. I am trying to get tech help from SSBC who made the kit but they are very slow in responding. If anyone has any ideas I would appreciate the help.

Hello Doug,
This may sound silly, but did you check to make sure the little output shaft on the booster is making good contact with the master? I changed booster on my last project and it would pump breifly but then lose pressure. No leaks no nothing just no pedal. Come to find out the little output rod was just barely pushing the master. Also, check to see if your prop valve is adjusted correctly. It may be causing unequal pressures in the front and rear lines causing the pressure loss and the brake light coming on. Just a thought.
Mark

Someone correct me if I'm wrong but, I believe the way it should be set up is with the prop valve to the front brakes before the dist. block. In this way it would act like a hold-off valve like on a factory setup. The reason I think this is that the discs need less fluid to operate whereas the drums need more. In a disc/disc setup you would put the prop valve inline to the rears so that you dont get rear lockup. I believe the reason you don't have a firm pedal at first is because you are having to pump the rears up till they cotact the drum.

Hello Doug,
This may sound silly, but did you check to make sure the little output shaft on the booster is making good contact with the master? I changed booster on my last project and it would pump breifly but then lose pressure. No leaks no nothing just no pedal. Come to find out the little output rod was just barely pushing the master. Also, check to see if your prop valve is adjusted correctly. It may be causing unequal pressures in the front and rear lines causing the pressure loss and the brake light coming on. Just a thought.
Mark
I wil check that ,Mark, Thanks.
Someone correct me if I'm wrong but, I believe the way it should be set up is with the prop valve to the front brakes before the dist. block. In this way it would act like a hold-off valve like on a factory setup. The reason I think this is that the discs need less fluid to operate whereas the drums need more. In a disc/disc setup you would put the prop valve inline to the rears so that you dont get rear lockup. I believe the reason you don't have a firm pedal at first is because you are having to pump the rears up till they cotact the drum.
The instructions that came with the kit state to install the prop valve after the distribution block in the line to the rear wheels. Thanks.

Prop valve goes on the rear line. Rear drums will lock too early with full pressure. A disc/drum master cylinder is different in that it already handles the volume difference. It does not account for the pressure difference, though.

It also gets installed after the distribution block because that detects a difference in pressure between the front and rear lines. If you put the prop valve on before it, you will cause a difference in pressure and activate the brake warning light.

The problem you describe is exactly the symptoms I had with a bad master cylinder. People have gotten bad masters from the parts store, so just because you just replaced it doesn't mean it's good.

Tim.

Prop valve goes on the rear line. Rear drums will lock too early with full pressure. A disc/drum master cylinder is different in that it already handles the volume difference. It does not account for the pressure difference, though.

It also gets installed after the distribution block because that detects a difference in pressure between the front and rear lines. If you put the prop valve on before it, you will cause a difference in pressure and activate the brake warning light.

The problem you describe is exactly the symptoms I had with a bad master cylinder. People have gotten bad masters from the parts store, so just because you just replaced it doesn't mean it's good.

Tim.
Thanks, Tim. I'm going through the steps that the SSBC tech help person is asking me to try. Hopefully, if it comes to the conclusion that the master cylinder is bad, they will stand behind it and replace it.

The tech at SSBC told me by e-mail that he is sending a new master cylinder.

Cool, good luck.

Tim.