Did your mechanic check the vacuum power booster? This is often the problem on these trucks. Here is how to test it
VACUUM POWER BOOSTER
With a "T" fitting, connect vacuum gauge into vacuum line between engine and power brake booster. With engine at operating temperature, gauge should read 15-19 in. Hg at idle with transmission in Neutral. If reading is below specifications, stop engine, disconnect vacuum hose at power brake booster and cap open end of hose and open port of vacuum "T".
Start engine, and allow it to idle. If vacuum reading is still less than 15-19 in. Hg, engine is producing low vacuum and problem must be corrected. If vacuum is to specification, check plastic check valve, rubber grommet and vacuum hose connection at power brake booster.
With low engine vacuum corrected and/or leaking components replaced, start engine, and allow it to idle. Stop engine, and depress brake pedal, holding down a few seconds. If vacuum drops to zero, booster is leaking and requires replacement.