"Interesting" combo DD3/spring brake setup.

[Don4107]

Just brought home a 5B.  Yikes! Third bus on the property!

Had weak brakes so I limped it the 40 or so miles home very carefully.   Getting the brakes checked out and adjusted was first order of business.  Weak brakes appears to have been lack of adjustment.  Not finished restoring to stock config yet so not sure.

Found a 30/30 spring brake on drivers side rear.  It was pushing against the air bag bellows.  This was done by a roadside repairman.  Parking and service functions appeared to work on both sides, spring and DD3. Had the lock port line releasing the spring brake and the emergency line plugged. 

DD3 was in the bay but had lock port leaking, hence the roadside repair by PO.  PO drove quite a few miles, read years, this way and claimed worked fine.

No need to go into all the reasons this was a bad idea.  Unbalanced in more ways than one.

But for a purely academic discussion, if the 5B originally had 24" DD3s and a short 24" spring brake can existed, it might work.  Shortest so far that I see is 8.3" or 1.3" longer than DD3.  Need to do more measuring to see if that would fit.  Also interesting reading on Bendix site about brake chamber force and adjuster length.

Surely there is some good reason for not using the DD3 lock line to release the spring brakes. But since the 85psi on the lock line is adjustable, and the inversion valve gives the right control, would there be any reason to re-plumb anything else?

Again, this is only for discussion. I am not advocating anyone do this. 

[bevans6]

The MCI MC-5's all had 30" DD-3's so the idea of the 24" creating additional room won't work.  I have no comment on the other idea of using the inversion valve - if I was in the mood to retro fit spring brakes, I'd toss the inversion valve, add an anti-compounding valve, and do it right with a dual air tank system.

I mentioned in my other post that I bought new DD3 cans for my MC-5.  I'm not the kind of guy that engineers new braking systems, but I can maintain what's there not so bad...  So I just bought new cans, bolted them in, put in a new inversion valve and all new air lines, and good to go for another 20 years or so, I figure...

Brian