A. I don't why the
pressure won't build in the master cylinder after you swap it, but the same
thing has happened to me before. I found a way to get around it;
Comments: I recently learned a VERY useful trick when bleeding brakes on a motorcycle. It works like a charm! Instead of filling the master cylinder up and trying to build pressure to fill the lines and caliper and force out any air, do it the other way around. This is also a good way to get clean fluid into the entire system. Here's what to do:
Get a fairly long clear hose that fits over the brake bleeder on the caliper. I mean about 18 to 24" in length. Get a small wrench that fits the caliper bleeder screw. Get an old bicycle pump with the T handle and a hose. Get an old turkey baster or another clean length of hose you can use to empty excess fluid as it rises in the master cylinder (or to suck up all the dirty fluid that is expelled into the master from the system so it doesn't get re-used later on).
Get at least 1/2 quart of DOT 3 or DOT 4 brake fluid.
Put the small wrench on the bleeder, then attach the long clear hose over the bleed screw. Fill the hose CAREFULLY with about 10" to 12" of fluid. Then put the hose end on the bicycle pump. Open the bleeder and put just enough pressure on the pump to force the fluid into the caliper and up into the lines. Stop before the fluid is all gone from the tube. Close the bleeder. Go back up top and GENTLY and SLOWLY work the handle for the brake until you get all the air out of it that will come. Repeat as necessary. (Be sure to cover the fuel tank so that if any fluid drips out the paint is protected. Brake fluid is a marvelous paint stripper!)
Once the system is completely without air, you'll have no more problems with the master building pressure