Out of curiosity: Air loss on full brake application.

[PorchPeople]

I have searched past threads and haven't found a solid answer. I have just rebuilt my DD3 braking system (new diaphragms throughout, relay valve, inversion valve, added E3 application valve, and lines). I also checked and rebuilt a couple slack adjusters, lubed the rollers and s-cams, inspected splines, and adjusted the brakes (1/2 turn off). I've got just under 2" travel on my pushrods (a little long I know), and I lose 11 lbs on a full application with the bus maxed at 118 lbs. If I hold the pedal down I have no air loss for hours so I know there aren't any definitive leaks in the system. Does 11 lbs seem excessive to you folk?
Much thanks!
-Michael

[lostagain]

I don't think 11 psi is excessive. As long as you say there is no air loss beyond that. You should look up the California DOT pre-trip procedure for commercial vehicles. That will tell you a lot. 2" push rod travel is too much for manual slack adjusters. You can probably reduce that by not backing off a 1/2 turn. 1/4 turn is usually enough to get the shoes off the drums. Also make sure the angle at the clevis is close to 90* when the brake is applied. Bendix also has lots of literature on line to learn from.

JC

[Jon]

If the air loss is large it might be the capacity of the tanks has been compromised. Could be as simple as water in the tanks, or someone replaced the tanks with a smaller size. I don't think 11 PSI is out of what I would consider a normal range however.

[Utahclaimjumper]

 Check your tanks,, on one bus I owned I had a similar problem and found five gallons of oil/water in the "wet" tank, huge difference when drained.>>>Dan

[luvrbus]

DOT rules and regulations allows a 12% drop so @125 lbs that would be a 15 lb drop

[brmax]

I have a time with some tanks not having the drain in the bottom but instead on the side somewhere. No sense in that, they should raise the tank mount sometimes in the build if clearance is a problem.
The slack distance is a bit to much and so could use a wheel lift and check for dragging and clearance when adjusting again Is my recommendation.
have a good day there
Floyd

[Lee Bradley]

The more travel you have the greater the pressure loss.  Each brake chamber is a little air tank and the longer the travel the bigger each tank becomes.  I back my adjusters off a 1/4 turn.

[chessie4905]

With brakes adjusted and still having excess play, you may have worn camshaft bushings and / or shoe anchors. Also check hub bearings for excess play.

[Geoff]

I have seen brake drums so thin that applying the brakes made the drum expand so much that the push rod travel would exceed 1" travel.

--Geoff

[TomC]

Adjust the brakes for 3/4" travel (1/3 turn out from tight). I bet your air consumption will be lower. Plus-how many times do you floor the brake pedal? (hopefully never). As long as the gauge doesn't continue to go down and stabilizes when holding the pedal, you're alright. Good Luck, TomC