Brake foot valve? not stopping well

First thing to check is does it leak at all? Air up, watch you gauge.....wait 5 min or so, did the needle move? Push half way and hold.....after the initaial drop, does it drop at all? An air leek hear could prevent the full pressure getting to the brakes. Do you have an brake application gauge that shows how much air is on the brakes? thia would let u know for sure.

 

Glazed brake drums will do this also, ck. closely

 

This is an idea, but im no mechanic, so...just an idea....

You said you had a couple. Are any of them the exact same model?

Can you press all the way down and up 10 times and see how much the air drops in the good truck (aired up but then turned off) Do the same in the problem truck. If it looses the same air, its likely a brake problem and not a valve problem. if it looses less, I would think the brakes arn;t getting as much air and thus we can confirm it is an air problem.

 

How hard would it be to put an air gauge on one of the air lines going to the brakes? Even just temporarily, to see what kind of pressure is actually making it to the brake chambers

 

Thats a more accurate idea....I was thinking that, but didnt want to break open any of the lines. Clearly we need to see how much psi is getting to the brakes though to determine if its an air problem

 

It sounds like the soft brakes your experiencing are from a worn out foot valve. But if you would like to check appication pressure on the service side of your chambers its simple. Being that you have multiple trucks that are the same, this " tool " will be used over and over. Get a short section of air line, a foot or so, that matches the size and fittings of your current service lines. You will need to have an air pressure gauge plumbed in the center of the line. You will unhook your service line at the chamber and comnect it to the short line, then the other end of the line can be attached to the chamber. Have someone apply the service brakes and you will have an accurate reading to that single chamber, test each chamber, noting any differences.

 

How do you have mushy air brakes?

Air compresses.

I would just replace the valve as a precaution.

 

Hello fellas. i've been here for a couple of months and love this forum and is really helpfull.
I need some help!
I own Volvo VNL 2011- cummins isx 500. In last time on my truck i feel like my brake pedal is going soft after hitting the brake once or twice. what it could be ? I let the truck build pressure and when the governor cut the air compressor i shut off the truck relased both braked yellow and red and there is no air leak, but right away i hit the brake and released right away air pressure drops to 100psi. under my brake ecu i have this code MID 136 SID 77 FMI 2 this code shows up together with TCS light and i hope is the same issue. I also have noticed when governor kick out the compressor while the truck running then i can hear still leaking air coming somewhere from the air dryer( new air dryer couple moths 3 moths ago). I also have gauge that shows how much pressure I'm applying to the brakes sometime when i try to stop it goes all the way but the truck barely stops. please help guys ( truck has 890000miles on it. brakes shoe and drums are never changed, maybe that is the issue?). thank you!
( btw when i bought the truck and turn the key ignition ON i could have air shooting from under truck before i start the truck, and now that is gone since that TCS light came on.