TB is right, If the reaction valve in the booster is cracked or busted. But notice he says the pedal took off on its own, and the engine was running.
Inside the booster is a very large powerful spring. Part of its job, is to return the control valve inside it, to its resting, un-applied position. In doing this, it also returns the pedal rod.If for some reason the valve doesnt return, then neither will the pedal. So the question is; Does the pedal return with the engine off, and the booster purged of vacuum, or does it not matter at all? The vacuum in the booster has to be purged, for this test to work.
If the pedal does return, with the booster purged, then the spring is ok.Pump it up several times just to satisfy yourself. Let the pedal return to the top. Start the engine. Does the pedal remain at the top?
1)If no it falls down, then the booster is faulty or the atmospheric valve is open. This could be due to a too-long pushrod adjustment. The easy fix is to loosen the booster retaining nuts and move it away from the firewall about 1/8 inch and retest.If you get the same results, get a different booster. But if the pedal now remains at the top, then shorten the pushrod.And retest
2)If yes, the pedal remains at the top; Put a wee bit of pressure on the brake pedal.Put a little more on it. If at any time it takes off and stays on the floor, Then the control valve is faulty and by definition the booster is junk.
But if the booster proves to be ok,and the pedal is not sticking, and the pedal still wont return, then;
first check all your rubber lines for clamps. Sometimes in the past , to isolate problems, I have had to clamp the lines. If the fluid cant return to the m/c, then the pedal wont return either.Perhaps your paid help forgot to remove a clamp.
After that check;
A) When you lift the pedal back to the top,does the pedal flop right back down again as soon as you release it? or B)Does the pedal stay at the top?This would indicate a possible pedal stiction problem
If A), the pedal flops right back down, That indicates the pedal assy is ok, and not sticking in any way.It also tells us that the piston in the M/C is staying in the bottom of its bore.So lets figure out why.The short answer is its stuck there, ei its no good.But there are two reasons for its not returning.The first is mechanical.Its physically stuck. The second is hydraulic.So Heres the hydraulic check:
I would check for fluid returning in the m/c. This requires a helper, and is done with the engine turned OFF/NOT running..First put rags all around the m/c to a radius of 1.5 ft, and extra layers towards the windshield,over the cowl. Then pop the m/cap.Have your assistant stroke the pedal down about 1 inch, and then very SLOWLY let the pedal back up.If the pedal sticks down, he will have to put a toe under it and lift it up, SLOWLY.While he is doing that you will be watching the fluid in the reservoirs. You will be looking for a little fluid roiling or even a little fountain, which is indicating the fluid returning. If at 1 inch stroke no fluid is seen to be returning, stroke again but adding 1/2 inch, again SLOWLY returning the pedal. Returning the pedal slowly is important because if the fluid is allowed to return too quickly, the fountain can be a gusher. When that occurs the fluid can spew up and hit the underside of the hood. Of course on its way back down it falls everywhere except back into the reservoirs. So remember-SLOWLY.Continue adding stroke until you see the fountain.This will prove that A) the m/c is working, and B) the seals in the calipers are doing their seal-retraction function, and C) the flex hoses are open, and D) the rear shoe return springs are on and able to do their job ie, the wheel cylinders are not frozen.
There is a slim chance that the fountain will not appear. Very slim. But if there is no fountain, now you have 4 possible reasons why. I would start with looking in the rear drums, then the flex hoses.In the end, you may discover that the short answer was right all along. But now you have proven it, have learned a bit about how it works, and are not just replacing parts with the hope that the problem will somehow solve itself, and are spending money once.
Happy hunting!
