My factory setup 76 LeSabre had a pedal that went to the floor when at a stop. After getting up to speed again(no where to pull over so had to get moving) the brake light illuminated on the ICP and I managed to get it to stop again up the road in a safer location by pumping it a few times. The light and issue stayed on until I parked and shut off car. No leaks, reservoir is full of fluid, and there is no leaks between the booster and the MC. I did notice both MC cover "boots" were fully extended all the way down into the MC but fluid was full. I retracted them back into the cover and reassembled...now there is nice solid braking and the brake light is off. I know there can be internal leaking in the MC, but all symptoms are gone at this time and I did put another 5 miles of test driving with about 10-11 "lock'em up" stops. Is it possible the "boots" were impeding braking somehow?
No, It's a master cylinder leak. One side of the piston (either front or rear), lost pressure and the pedal went to the floor, the fact that one side was not holding pressure caused the light to come on and the boot to suck in. Because the leak is internal to the master, the fluid stayed put. Assuming you have checked all the flex hoses and they are good(ie not ballooning), I would replace the master.
Yep. If the boots were pulled all the way down, and the reservoir halves are full, I am thinking internal leak, as it would move the greatest amount of fluid. In a sense a "closed loop bleed". In the USAF we used hydraulic "Mules" a three system supply and return lines connected to each hydraulic system on the aircraft, each with a supply (pressure) and return (suction) line, and the mule provided 3000 PSI at high volume to pressurize the systems and allow flight control, gear operation for rigging, testing and troubleshooting. The flow could be open loop or closed closed loop. Allowing the test stand and aircraft systems to freely flow fluid, or in closed loop (stand only) used to bleed the test stand (mule) before going to open loop. A Master Cylinder with an internal leak, is like that mule in "stand only" closed loop and moving the fluid from the system reservoir, through the pumps and lines (but not the hoses) and back to the reservoir. That could move enough volume at a full stroke to pull the bellows (boots) into the reservoir, and result in fluid level being full.
Yep, just replaced the master on my ‘68. It would go to the floor once in a hundred stops and be fine the next 99. No leaks anywhere. Had the same issue with a Trans Am twenty years ago; 1/100 becomes 1/50 becomes 1/25… Patrick
Had one of my technicians replace the master cylinder and both wheel cylinders(one was starting to leak and other had trace seeps). Test drove for about 25 miles and then put them thru the ringer with a 3500lb trailer in tow between MN and WI this weekend. "Beta test" for future trip to Nevada doing the same thing passed with flying colors.
