This is a strange problem for me and I hope someone else has experienced it and can help me out.
With the engine running, sometimes when I press the pedal it will drop alarmingly low before any braking happens. If I then lift up and press it again the pedal and booster act normally and braking is fine. My initial suspicion was that there is air in the lines, but when I am parked with the engine off the brake pedal is perfectly hard. My next thought was that I am losing fluid into the booster, but the level in the reservoir is not going down. I would like to know what I am looking for before I tear into the booster. Does anyone have any advice as to what would cause these symptoms?
I would check the integrity of your flexibles, Mike. I’ve had a similar experience with the bride’s 72 XJ6. Usually after a long freeway run, brake pedal would sink very low and return to normal for next application. Problem was the hose between the remote reservoir and the brake master had swollen internally interfering with fluid travel. Incorrect hose installed by your’s truly several years prior. FWIW.
I’ve recently had the same issue as Paul, on one of the front fl I left hoses.
Well, it turns out that there is only one place in the braking system that will allow the brake pedal to drop nearly to the floor without losing brake fluid somewhere, and that is inside the master cylinder. If the seal in the end of the piston doesn’t seat properly the fluid will simply be shot back into the reservoir when the pedal is pressed. Rebuilding the master cylinder solved the problem. It seemed to me that I had already done that recently, but upon reflection it was probably at least 20 years ago. Time seems to go faster the older I get. Just for good measure I’ll replace the hoses too.