Help!! Automatic Brakes

[turbofoot1]

Thought my master cylinder was bad cuz it was sticking at the top, so I replaced it with a known good used one off my other 86 t-type. Now, When I turn on my key, I hear the powermaster start running and my brake pedal begins slowly moving to the floor by itself. When the powermaster finally shuts off, the pedal remains on the floor and the brakes are fully applied. NO foot assist at all. All automatically performed by the Powermaster, obviously. I have tried 3 pressure switches and ohm'd each one open and closed, they are fine. The brakes work normally in unassisted (disconnsected PM) mode, which leads me to believe master cylinder is fine. I have tried 3 accumulators off same car, no difference. I have swapped all my parts around every combination and end up with the same results. PM still applies brakes when key is on and car won't move. I am a mechanic by trade and at 50 thought I was pretty decent at diagnostics but have fooled myself again. Any help would be greatly appreciated. Rusty

 

[2badgn]

Thought my master cylinder was bad cuz it was sticking at the top, so I replaced it with a known good used one off my other 86 t-type. Now, When I turn on my key, I hear the powermaster start running and my brake pedal begins slowly moving to the floor by itself. When the powermaster finally shuts off, the pedal remains on the floor and the brakes are fully applied. NO foot assist at all. All automatically performed by the Powermaster, obviously. I have tried 3 pressure switches and ohm'd each one open and closed, they are fine. The brakes work normally in unassisted (disconnsected PM) mode, which leads me to believe master cylinder is fine. I have tried 3 accumulators off same car, no difference. I have swapped all my parts around every combination and end up with the same results. PM still applies brakes when key is on and car won't move. I am a mechanic by trade and at 50 thought I was pretty decent at diagnostics but have fooled myself again. Any help would be greatly appreciated. Rusty

Check the proportioning valve on the drivers side of the front chasis.

 

[zeus87gn]

What did you do when you bled the unit that you installed?

It was a known good unit, there had to have been something you overlooked.

 

[turbofoot1]

Through more testing, I have discovered the pm works normal until I fill and attempt to bleed the master cylinder. Do they need bled in any certain order?

 

[zeus87gn]

Yes. It needs to be set up correctly.
Check my procedure at the link in my signature.

 

[turbofoot1]

Still no luck. If I hook up either of my Powermaster/master cylinder assembly units to the electrical connections and fill just the P/M side of the reservoir, as soon as I poer them up, they depress the master cylinder all the way in. This occurs on both units and on 2 different cars. Hydraulic pressures this high 500-700psi have to be metered or controlled inside the master cylinder somehow and only allowed to apply light pressure on demand. I don't know how this is controlled and have found no technical data that explains it.

 

[zeus87gn]

Turbofoot,

Sorry for the late response, but I had to think about this for a bit. I have only heard of this once and there was never a solid solution. I was not part of the conversation.

The only way I can think of that this could happen is if the valving inside the power piston (internal component) is hung open. Once you get it filled up, power up and the pedal goes down, try shutting it off and push the pedal to see if it may free up the stuck valve. Try tapping, maybe use a little more force, try a good firm push, get more aggressive until you feel it either free up or you think it's not working.

Something I did notice just as I came back online - You should have both sides of the bowl filled, or at least some fluid in the d/s, to keep things in order within the master cylinder assembly. The backforce that it would provide might be what that valve is looking for.

Please post back and let me know if you succeed.

Z