How much air pressure should I be making at idle?

[Paladin]

Today I started the bus after having it sit while I welded the plate over the air beam.

The pressure came up pretty quickly but at about 85 psi it stopped and then began to drop a little. It held at about 70 psi and then climbed a little and held at about 80. I held some pressure on the throttle and it came to about 85 and finally about 95 which was when I shut it down since it was evident that it was building at a higher throttle level.
I listened for air leaks and there was a slight leak from the left wiper valve but after I bumped that it was stone silent in the bus even with the floors all up. No hisses, no noise at all. Even my repair on a line in the tunnel is holding tight. I swear it came up slower but higher at idle when I ran it a couple of months ago. It was building but leaking out then.

How much air pressure should I have at idle? With more throttle it came up and now that's it's been off for about 20 minutes it's lost about 3-4psi although that seems negligible for the slow rate of build.


-Dave

[Kwajdiver]

Dave,

My bus has a very little leaks,,, but I have no trouble building up to 120psi.

MCI-9
Bill

[Paladin]

Update:

I just went out and tried it again and this time the pressure went pretty smoothly up to 120 and then stayed at around 110 - 120. It did drop to 100 for a minute or so and then went back up.

So, what is the normal pressure at idle?  I wonder why this time it went up higher and held?
If I'm getting 110-120 now and if it stays that way can I assume that aside from my wiper valve leak I'm pretty solid? 

-Dave

[Stan]

You are mixing up pressure and volume. The max pressure is set by the air governor on the compressor. If it is set for 125 PSI the compressor will pump continuously (regardless of engine speed) until it reaches that pressure and then cut out. The volume is determined by how fast the compressor is turning. More engine RPMs equals faster air build.

What you were watching earlier was the system has an brake protection valve which does not allow any air to go to the air ride or the auxiliary tank until the brake system is fully charged, which on your bus appears to be about 80 PSI. When you went out the second time, the air ride and auxiliary tanks were still full so the pressure appears to rise at a steady rate.

[Paladin]

Ahh......makes sense. Thanks!