Chasing air leaks

benherman1 · June 26, 2020, 08:39:08 AM

Hello,
I have a quick question relating to a possible leak in my rear brakes. I just replaced my main brake valve (thanks Luke!) and can now hear the other leaks it was hiding. Among others there seems to be a small hiss only when the brakes are fully actuated. While typing this I now realize I also forgot to turn off the parking brake while this was happening. Should something hiss when pressing the brake with the parking brake on? Did I damage anything doing so? I feel like I read somewhere you aren't supposed to do that but don't remember for sure...

luvrbus · June 26, 2020, 09:27:35 AM

it should have a anti compound valve in the system to prevent damage that is probably were the hissing was coming from

buswarrior · June 26, 2020, 10:02:20 AM

"Main brake valve" ?

Can't help you if you don't use the proper terminology. It would ha ve been on the box.

If the parking brake releases next time, then no harm done.

Happy coaching!
Buswarrior

benherman1 · June 26, 2020, 10:18:15 AM

I just aired it back up and tested again with the parking brake off. Its a different noise this time but still a significant hiss from the passenger side brake can. It seems to be coming from either the hose going to the top of the can or the can itself. I'll assume the hose until I get that replaced since they are all ancient and well past being safe to drive on. It's nice to know it has the compound valve for when I forget what I'm doing...

benherman1 · June 26, 2020, 10:23:10 AM

Quote from: buswarrior on June 26, 2020, 10:02:20 AM
"Main brake valve" ?

Can't help you if you don't use the proper terminology. It would ha ve been on the box.

If the parking brake releases next time, then no harm done.

Happy coaching!
Buswarrior

Unfortunately it came in an unmarked bag so no help from the box. It is the "foot valve" or "treadle valve" depending on who you are talking to. It is the "main" brake valve as it applies air to the braking system.

jap42 · June 26, 2020, 12:43:40 PM

It would probably help to know front or rear. The treadle valve is the main control of the air. But the relay valve supplies all the air to your rear brakes. I think in all circumstances the front braking comes directly from the treadle valve.

Parking brakes are typically only on the drive axle. I had to learn about this parking brake system from some of the guys here, I have always dealt with spring brakes. The issue with applying the brakes with the parking brake applied is the parking brake is held by a lock that is held by the return pressure from the brake shoes being applied. Which is why they continue to hold without air present. The problem is in order to remove that lock you have to apply more pressure then the parking brake was set with. So if you mash the peddle down with that lock engaged you could lock the cylinder permanently (Or until you can connect to an external air source and increase the pressure beyond what you need to disengage the brake.)

benherman1 · June 27, 2020, 07:46:05 AM

Quote from: jap42 on June 26, 2020, 12:43:40 PM
The problem is in order to remove that lock you have to apply more pressure then the parking brake was set with. So if you mash the peddle down with that lock engaged you could lock the cylinder permanently (Or until you can connect to an external air source and increase the pressure beyond what you need to disengage the brake.)

That sounds like what I read that made me worry I had caused myself more trouble. Luckily when I went to go test again it looks like they released normally and all is well there. When connecting to external air I've been setting the regulator at 90-100 psi so if I do something dumb there should theoretically always be the option to fire it up and fix it.

My leak is in the rear which is why I was thinking it could be parking brake related. I think I need to rule out the air lines before I do anything else. They're pretty awful. I went under while it was aired up and had the girlfriend hit the brakes a few times and I could see them swell where there were cracks in the outer casing.

chessie4905 · June 27, 2020, 07:56:48 AM

Good thing is is air hoses are cheap compared to most everything else. HD truck parts dealers carry them. Also NAPA. Just match length and fittings. Be careful to mark them, as to not mix them up, especially on the DD-3's.