Air Brake Pressure Loss WITH Pedal Applied

[MyRVButler]

I'm experiencing a leak in the rear air brake chambers that looses PSI with service brake pedal applied (air suspension stays fully inflated only the rear air tank that leaks).  In other words, applied air brake system leak, it's expelling too much PSI when pedal is depressed.  I guess over time if the brakes aren't used for a bit the bladder inside chambers can deteriorate and leak? Matt

[ktloah]

Yup, sounds like you have a bad brake chamber...

[MyRVButler]

Agreed.  No air leaks when coach is static after it pops off.  Its only with pedal apply, and the pedal feels weird, soft and it kinda drops when you give it a firm press so has to be one of the chambers blowing out.  Chambers aren't that pricey, MGM brand common at my local RV chassis shop. 

[Ivan K]

Not that unusual and easily repaired without even having to readjust the brakes, unless you go with new canisters. Just confirm that it really is where the leak occurs. It could be mechanically worn out or deteriorating due to oil and water in tanks. The replacement rubber pancakes are just few dollars and the repair takes minutes for a shop. On drive axle with spring brakes one needs to be careful and cage the spring and only undo the service part clamp.

4 minutes ago, MyRVButler said:

Agreed.  No air leaks when coach is static after it pops off.  Its only with pedal apply, and the pedal feels weird, soft and it kinda drops when you give it a firm press so has to be one of the chambers blowing out.  Chambers aren't that pricey, MGM brand common at my local RV chassis shop. 

When you say the pedal drops, it does not mean the chambers are leaking at the rear. The pedal feeds a relay valve, not the chambers back there so you need to identify the leak before diving into replacements. 

[MyRVButler]

Well, I keep performing COLA Brake Test every time I fire up.  After pop off, I depress service brake, turn off motor, release Spring Brake and watch the guage, it drops way more than 3lbs.  A couple secs later I hear air expelling, like I'm lifting my foot but I'm not, thats what I mean by the "pedal drop".  Applying further just increases the rate of air psi loss.  Maybe leak at air relay, thats the valve assembly behind pedal assembly, on exterior firewall in front cap. 

[Ivan K]

The point of relay valve is to be close to the axle and air tank so that's where it should be and seems like it is releasing the brakes when the signal pressure from pedal drops due to a leak somewhere between and including the pedal assembly and relay valve.

[Bob Blackmon]

The treadle valve only operates the relays that supply air pressure to the cans. If you can feel a difference in the pedal you may want to look there.

[Cubflyer]

Does anyone know what pressure the switches at the brake pedal (for lights, etc) are designed to operate at? Is it zero pressure when pedal is not pressed? and 10,20,30 or more psi (120psi??) when pressed?  What would the switches "see" for pressure under normal braking?  Under heavy braking?  I'm trying to source brake light replacement switches...  I've found a switch that goes from NC to open at about 50 psi and NO to closed at about 60 psi... would it work??

Thanks for any knowledge on this..

Ken

[Ivan K]

I know my 2 wire cruise cancel switch is NC 5psi,  brake light switch should be the same but NO. Unless you have 3 wire switches combining the 2 functions.

[Cubflyer]

3 hours ago, Ivan K said:

cruise cancel switch is NC 5psi

So the brake 'control' air pressures are pretty low relative to the system pressure...probably 0 to 50 psi...??  ... sounds like I need to look for much lower pressure activation switches.    Thanks Ivan!

[Ivan K]

3 hours ago, Cubflyer said:

So the brake 'control' air pressures are pretty low relative to the system pressure...probably 0 to 50 psi...??  ... sounds like I need to look for much lower pressure activation switches.    Thanks Ivan!

Ken, my switch is equivalent of this one, don't know if yours is similar. Rated 3-5 psi. Cross-reference to Haldex/Wabco/Meritor are multiple times more expensive.