Parking brake won't release, some advice needed.

[scanzel]

I have an 89 Prevost and went to move it away from the trees before Irene hits here and the parking brake won't release. The bus airs up ok but when I release the brake pushing the brake release button in the air pressure starts droping guickly. When I turned the engine off and attempted to release I could hear air escaping in the back by the drive wheels. I don't have any time to investigate further with Irene coming but is there anything particular I should look for when I can get under to investigate the rush of air ?
Thank you.

[robertglines1]

Steve. If you must move just hold the valve down and drive thru the brakes. Also ck see if on your left console to the rear you have a green knob. emergency brake release.override.  try it. it won't last long so be prepared to go.    when this passes ck pancakes and air lines. you can prob locate by air noise or elimanate with wise grip clamping off hose to figure out which one. crossing fingers for you all.   Bob & Judy

[bobofthenorth]

Sounds like a relay valve Steve - between the drives, up against the floor with about 6 hoses coming off it.  Its probably too late now to move the bus but you could likely cage the springs on the drive axles and get it to move around your yard if that's all you need to do.

[buswarrior]

Relay stuck, or a hose has burst, or the spring portion's diaphragm in the brake chamber has ripped.

You'll easily find it once you get under there, safely of course.

happy coaching!
buswarrior

[prevosman]

Sounds like a bad diaphragm on the spring brake. Temporarily it can be caged and the air line to the emergency brake side plugged to make it driveable.

[scanzel]

I am going to try and look at it this week after Irene yard clean up. What do you mean by caged, not familiar with the term. Thank you.

[bobofthenorth]

"Caged" means that the parking spring has been compressed by inserting a t-bolt and tightening it down such that the spring is held in the compressed position.  Effectively you are substituting the bolt for the air pressure that normally holds the spring off or "cages" the spring.  Occasionally on an older vehicle the t-bolt will still be in its bracket on the brake can but usually someone will have borrowed it along the way. 

[bevans6]

Did Prevost's always have spring brakes or did they switch from DD3's at some point?

[luvrbus]

No Prevost switched over in the 80's the early H-40 some had DD3 and those didn't hit the market till 1989,the 1982 I looked at had DD3 brakes

good luck