Good day! I own a 2004 toyota corolla. I have a spongy brake pedal that hits the floor with brake failure when stepped on for long like when in a traffic jam. The brake works perfectly when pumped and with mild, short interval pressure on the brake pedal. Brake fluid level is adequate. The caliper has been serviced once when it was noted that the brake fluid was leaking and its rubber ring/seal changed. Master cylinder's seals been changed multiple times. Replaced the master cylinder few months ago with hardly no change. Brake fluid has been bled multiple times. Brake pad is still good. Any suggestions on possible cause and how to go about repairing it? Been managing it for some months now as it has defied the expertise of my mechanic. I appreciate any help.
Treat this as a new problem, and forget all the repairs that have been made.
The first step is to eliminate the possibility that there is a brake fluid leak. Carefully and accurately mark the brake fluid level on the reservoir with a pencil, and then get in the car and force the brakes to "fail", as you say, by pressing on the brake pedal for a long time. Repeat this step several times.
Now check the fluid level. If the level has dropped, you have a leak and you must find it. If your mechanic can not find it, get a new mechanic.
What if the brake fluid level does not change? It's not rocket science: the only other possible cause is a defective brake master cylinder.
No matter how many times you have rebuilt the master cylinder or replaced it, if the fluid level remains constant but the pedal drops to the floor when you press and hold it for a long time, you have a bad master cylinder. There is no other rational explanation.
Install a good master cylinder and your problems will be over. If your mechanic has been getting rebuild kits or a rebuilt master cylinder from a questionable source, or if the rebuild kits were intended for a different model of master cylinder, this can explain the repeated failure to fix the problem.
Also, if one of the cylinders in the original master cylinder was scored (scratched) from abrasive particles in the brake fluid, any number of rebuilds will fail to fix it.
You might need a new, not rebuilt, master cylinder direct from Toyota.