* Vacuum leak in the brake booster: The brake booster uses engine vacuum to assist braking. If a vacuum line was disturbed during the transmission work, it could have developed a leak. This means the booster isn't getting enough vacuum to assist, and the added load of the engine dropping RPMs (or stalling) when braking pushes it over the edge.
* Damaged or improperly connected vacuum line: This is closely related to the vacuum leak. A line might be pinched, cracked, or simply not connected correctly after the transmission work.
* Throttle cable adjustment (if applicable): Improper adjustment of the throttle cable can lead to an idle that's too low. When you brake, the engine drops its RPM even further, leading to a stall. This is less likely with electronic throttle control but still possible depending on the specifics of the truck.
* Transmission linkage or control issues: While less common with modern electronic transmissions, if the transmission wasn't properly installed or adjusted, it's possible the shift linkage is interfering with something else, potentially causing the engine to stall under braking.
* Incorrectly adjusted idle speed: The mechanic may have not properly set the idle speed after the transmission work. A low idle leaves little margin for engine load increase when braking, resulting in a stall.
* Electrical issue (rare): Although less likely, a short or faulty connection related to the transmission's electrical components (sensors, solenoids) could interfere with the engine's operation causing a stall. This is more probable with electronically controlled transmissions.
* Low fuel pressure: Though less directly related, low fuel pressure can cause the engine to struggle and stall when put under load, such as when braking.
What to do:
The problem requires professional diagnosis. Don't attempt to fix it yourself unless you have significant mechanical expertise. Return to the mechanic who performed the transmission work and explain the problem. They should be able to diagnose and fix the issue, as it likely relates to something they worked on. Clearly describe when the stalling happens – specifically mentioning it's *only* when braking. This significantly narrows down the possibilities.
