suspect parts imo considering the age of your car and assuming no brake jobs before:

1. internally split brake hose (pressure is contained and is not pushed back to the master cylinder when released)
2. piston seals worn out and lost its "spring action" (won't push the piston back to its released position)
3. master cylinders (rubber caps may be expanding and covers the compensating port when the brake fluid is heated)
4. proportioning valves/metering valves etc - might be out of proportioning action

brake booster is a remote possibility of locking up the front.

try mo na lang sa ibang service shops. wag ka na bumalik sa dati mo mechanic obvious na hindi sila marunong at gusto lang perahan sa back jobs.