QuestionQUESTION: Hi Barrie, per an earlier discussion, I just replaced the calipers on my '80 B. I'm having a problem bleeding the rear brakes. The fronts bled out no problem, but when I bleed the rears, I get nothing but an apparently infinite supply of air. I've tried both a MityVac and a simple little bottle, with the same results. We pump the pedal for like half an hour, and air just keeps coming out. The reservoir is full, and doesn't drop significantly during this process. I was thinking there might be a leak somewhere, which would make sense if I were just using vacuum, because then I could be pulling in air, but since I get the same results without vacuum, I kind of ruled that out. Thoughts? Thanks!
ANSWER: Hi Billy. Did you undo the pressure differential switch 3.5 turns before trying to bleed the brakes? This is essential. Your system has a front/rear dual master cylinder, and the switch can cause the differential piston inside it to stick, making bleeding impossible. If you don't know where it is, follow the pair of wires going to a threaded switch in the middle of the master cylinder (underneath it).
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QUESTION: The switch is history on my car. So when I replaced the master cylinder, which I did some time earlier, I plugged the switch hole, making sure to use a plug short enough not to touch the differential piston. At that time, the system bled out fine.
I wasn't sure if fluid even flows to the switch hole, so I plugged it just in case; should I go ahead and remove the plug (at least partway) and try bleeding?
Answer
MGB Tandem Master Cyli
If the switch has been removed and the hole plugged, that should work. I assume the front brakes are working OK but the rears have no fluid in them. Somewhere along the line you have a blockage, or else the seal on the end of the master cylinder piston has failed.
The attached exploded diagram may help.
By the way, are you CERTAIN that the master cylinder reservoir is full? It has two compartments, and it is possible for the front one to be empty while the rear one is still half full. Since the front one supplies the rear brakes, you could be pumping air.
