I have two very similar fire trucks. An '81 C8000, and an '88 C8000. Both have the same Cat diesel engine and Allison transmission.

My problem is with the brakes on the '81. They take a minute to a minute and a half to pump up to 60-70 psi to release....Then, after you drive it, it goes all the way up to the 130 range, at which point I hear an apparent relief valve going off every 10-15 seconds or so.

Sounds like it's "regulating" itself by the relief valve.

The '88 is just fine. Pumps right up, and maintains between 110-120 psi with no issues or relief type noises.

Your thoughts?

 

We need to know what engine you have to better help. If i was to guess it would be the "D2 governor" which is mounted atop the compressor usually but, thats just a guess.

 

Cat 3208. both trucks have identical engines.

If it matters, the "good" truck holds pressure for days while not used. The problem truck leaks off within hours.

 

I would start off by fixing the leaks in the system.
Let it build pressure then shut truck off, with a squirt bottle with soapy water in it spray every air line fitting you can find. Starting at the compressor and working toward the tanks. Not uncommon for fittings to leak after 20plus years of service. Be sure to check drain valves on both tanks also.
Is one tank leaking off faster then the other? Can tell by air gauge in cab should have 2 needles (one for each tank).

Good luck let us know what you find.