I am having some issues with my brakes on my 79 Hurst. End of last summer was braking and pedal went to floor brake light came on. This happened again this year. I replaced the control valve. Fluid is full, and no fluid leaking. Rubber vacum line and metal line to carb have no leaks as I can see. When car is in gear is when it goes to the floor and brake light comes on. In park or neutral brake feels fine no brake light comes on when brake pedal is pressed. Thanks any info would be great. Brian

 

If no leaks, no air, I would look at replacing the master cylinder....

 

X2 on the master cylinder if there are no leaks and the brakes are adjusted correctly. Normally a loss of vacuum leads to a hard brake pedal.

 

Not sure about this since it is only happening when in gear, but I would also check the rear drums, specifically, the adjusters and the lining thickness. Also make sure no wheel cylinders are leaking. Good luck.

 

Why?

As others have said, if the pedal goes to the floor without losing fluid, it's time for a new master cylinder.

 

This is a classic symptom of master cylinder failure.

Inspect the complete brake system, if nothing is excessively worn or leaking just replace the master cylinder. Not much else that can cause the pedal to sink to the floor.

 

Check the back of the mc, it may be leaking there. Had that happen once, utter and total brake failure, got home on the parking brake and downshifting.