What am i looking for with the switch wire or switch am i looking to see of it is open or ground
You should see an open circuit when testing the switch to ground. Your brake light is always hot looking for a ground to complete the circuit. If there is a problem with either the front or rear brakes, the piston in the valve will move to that side providing a ground that sends a ground to your brake light. If you removed the switch and it shows an open, then the piston in your proportioning valve has moved to one side or the other. Or, you could have developed a ground somewhere on that black wire between the switch and the light. Or.....it could also be a ground from the parking brake circuit as mentioned in post 22.
What causes the piston to move to one side or the other causing pressure issue? Bad Master cylinder maybe or improperly bled brakes?
The piston moves to the side of lower pressure due to a hydraulic failure or leak (seals, etc). If you have a leak you should be able to see the fluid leaking, wet spot, etc. The fix is to open the bleeder on one side of the front or back and press on the pedal, then close the bleeder. If that does not do it, then do that same procedure on the other side you did not do and see if that fixes it.
Usually, it is a broken line and loss of fluid and or pressure to either front or back. I can't imagine a bad master cylinder causing it because they either push fluid or not.
To me, this scenario indicates that when you press hard on the pedal, the piston is trying to go back to center from which ever side it is on. Effectively opening the ground. I have heard that stomping on the pedal a few times can move the piston back but I have no proof of it. Worth a try though.
Perfect will do fluid bleeding today. I inspected everything and no leaks and everything look how it should
Thats exactly what is going on i did try the pedal trick did t work, today i will go to store and get brake fluid and bleed.
I had an issue years ago with a brass proportioning valve that the piston didn't want to move back to center after I had a rear brake cylinder fail and lost fluid to the rear brakes. I removed the switch and used a small pick to reach in and move the piston back. You may have some luck with that. It doesn't explain why it happened in the first place though.
We can use dot 5.1 with our stock systems?
I just bought car so who knows what is in there now. Will first bleed with dot 3 and see if i can get valve re centered before i purged everything
Ideally start with new components. Short of that make sure to get rid of ALL the old DOT 3/4 fluid as they are incompatible. The pedal feel will be different after converting to the purple stuff too!
I dont want to go through all the stuff that is good will just stick with dot 3 and get myself back to square 1
No problem. My car was a rotisserie resto and I did not want to accidently spill brake fluid onto that paint!
