BPepper Posted March 6, 2023 Report Share Posted March 6, 2023 My 2001 Volvo service brakes have recently started to not hold at stops. When I stop at a stop sign, red light, etc with normal brake pedal application the truck will sometimes start to roll either forward or backwards depending on the incline I'm on. If I mash the brake pedal it will stop and hold. I've been driving this truck for 3 1/2 years and its only recently started doing this. I've checked all the brakes and the lining is still good, slack adjusters are self adjusting but I did check and they are correctly adjusted. The parking brake holds good. Only the service brakes. Any ideas on what to check next? Thanks Quote Link to comment Share on other sites More sharing options...
Darryl&Rita Posted March 6, 2023 Report Share Posted March 6, 2023 When was the last time the adjustment was checked? What is your air consumption like with the brakes applied? Do you hear any leaks with the brake pedal pressed? Quote I have been wrong before, I'll probably be wrong again. 2000 Kenworth T 2000 w/N-14 and 10 speed Gen1 Autoshift, deck built by Star Fabrication 2006 smart fourtwo cdi cabriolet 2007 32.5' Fleetwood QuantumPlease e-mail us here. Link to comment Share on other sites More sharing options...
BPepper Posted March 6, 2023 Author Report Share Posted March 6, 2023 When the service brakes are applied my air pressure stays at 120 psi per the dash gauges. I just checked the adjustment last week. I don't hear any audible leaks when the brakes are applied. I was laying under the truck while my son applied and released the brakes with the truck off. Quote Link to comment Share on other sites More sharing options...
BPepper Posted March 6, 2023 Author Report Share Posted March 6, 2023 Just went and started the truck again. Now, when the service brakes are applied with engine running pressure is dropping to around 100 psi. Quote Link to comment Share on other sites More sharing options...
Nigel Posted March 6, 2023 Report Share Posted March 6, 2023 Are you in freezing temperatures? If so check the tanks for moisture and service the air dryer. Quote 2006 Volvo VNL 430, 2006 smart cabrio cdi, 2000 Triple E Topaz 30' Link to comment Share on other sites More sharing options...
BPepper Posted March 6, 2023 Author Report Share Posted March 6, 2023 23 minutes ago, Nigel said: Are you in freezing temperatures? If so check the tanks for moisture and service the air dryer. No, I'm in South Alabama, currently 77° out. Quote Link to comment Share on other sites More sharing options...
ddm502001 Posted March 7, 2023 Report Share Posted March 7, 2023 Could be relying on one of the two circuits under light applications. Have someone else LIGHTLY apply brakes with wheels Chocked and Parking brakes release, listen and look for actuation as brakes apply, may only be Steer or Drives on light apply where on Parking go full spring application. With parking applied, Slack Adjusters should not rotate beyond 90 degrees to Chambers. Quote Link to comment Share on other sites More sharing options...
Darryl&Rita Posted March 7, 2023 Report Share Posted March 7, 2023 (edited) 7 hours ago, BPepper said: Just went and started the truck again. Now, when the service brakes are applied with engine running pressure is dropping to around 100 psi. That's a signal. There has to be an air leak, somewhere, to see a drop like that. Diaphragm or air hose, or a leak inside the air treadle valve inside the foot pedal. Time to put some ears to it again. ETA: Along with what ddm... posted, are you using the dash knobs the same as always? Running in bobtail mode can decrease your effective braking. Edited March 7, 2023 by Darryl&Rita Quote I have been wrong before, I'll probably be wrong again. 2000 Kenworth T 2000 w/N-14 and 10 speed Gen1 Autoshift, deck built by Star Fabrication 2006 smart fourtwo cdi cabriolet 2007 32.5' Fleetwood QuantumPlease e-mail us here. Link to comment Share on other sites More sharing options...
BPepper Posted March 7, 2023 Author Report Share Posted March 7, 2023 I am using the dash knows the same as I allways have. What do you mean by bobtail mode? All I have ever done was release the parking brake and drive. The trailer supply knob is used to supply air for a bed lift for loading a smart car. I have only used it a few times to lift the bed to clean under it. All I have ever done when bobtail or towing is release the parking brake. Quote Link to comment Share on other sites More sharing options...
rickeieio Posted March 7, 2023 Report Share Posted March 7, 2023 I believe what Darryl is referring to, is that only pushing in the yellow knob, on some trucks, will only give the drive axles about half the braking force as when both knobs are pushed in. Our 2001 Volvo was that way, so it's likely yours is too. Did you buy the truck with the brake supply providing air for the lift, or did you build it that way? Quote KW T-680, POPEMOBILE Newmar X-Aire, VATICAN Lots of old motorcycles, Moto Guzzi Griso and Spyder F3 currently in the front row Young enough to play in the dirt as a retired farmer. contact me at rickeieio1@comcast.net Link to comment Share on other sites More sharing options...
BPepper Posted March 7, 2023 Author Report Share Posted March 7, 2023 I bought the truck that way. Quote Link to comment Share on other sites More sharing options...
Moresmoke Posted March 9, 2023 Report Share Posted March 9, 2023 Start truck, let air pressure build up. Turn truck off, key back on but do not start. Push and hold the brake pedal applying 80 psi on the application pressure gauge. After 30 seconds, the tank air pressure should not drop noticeably from the initial drop when you pushed the brake pedal. Also, any truck that was factory built as a tractor since the early ‘90s had a brake proportioning valve. This valve reduced the pressure to the rear wheels to help reduce the chance of skidding when bobtail. The red knob is the trigger for the proportioning valve. Quote Link to comment Share on other sites More sharing options...
BPepper Posted March 9, 2023 Author Report Share Posted March 9, 2023 So to activate full braking pressure I need to engage the red knob? Can this be done since the gladhands have been removed from the truck? Would the braking be noticeably better? Quote Link to comment Share on other sites More sharing options...
rickeieio Posted March 9, 2023 Report Share Posted March 9, 2023 Yes, push in the red knob and try it. Hopefully, whomever modified your truck left the important bits intact. The gladhands have no bearing on tractor brake function. Quote KW T-680, POPEMOBILE Newmar X-Aire, VATICAN Lots of old motorcycles, Moto Guzzi Griso and Spyder F3 currently in the front row Young enough to play in the dirt as a retired farmer. contact me at rickeieio1@comcast.net Link to comment Share on other sites More sharing options...
Jaydrvr Posted March 9, 2023 Report Share Posted March 9, 2023 (edited) If you still have gladhands, they need to be blocked off so they don't leak when you push in both air buttons. If your gladhands are removed and capped, it's already done for you. Push in both buttons and watch/listen for air leaks. Alternatively, you can replace the proportioning valve with a regular valve and avoid all that. Jay Edited March 9, 2023 by Jaydrvr Quote Link to comment Share on other sites More sharing options...
BPepper Posted March 10, 2023 Author Report Share Posted March 10, 2023 Where could I purchase a regular valve at? Quote Link to comment Share on other sites More sharing options...
Jaydrvr Posted March 10, 2023 Report Share Posted March 10, 2023 1 hour ago, BPepper said: Where could I purchase a regular valve at? Any truck repair and parts place will have that. Just explain what you want to do and have them supply you with the correct valve. I don't remember exactly, because it's been over twenty years and my mechanic did it for me. Jay Quote Link to comment Share on other sites More sharing options...
porky69 Posted March 10, 2023 Report Share Posted March 10, 2023 Sounds to me like you have a bad relay valve. I am all for doing it yourself but if you are not real familiar with how air brake systems work and are plumbed, take it to a truck shop..it's not just your life at stake it's everyone on the road. Quote Link to comment Share on other sites More sharing options...
rickeieio Posted March 11, 2023 Report Share Posted March 11, 2023 8 hours ago, porky69 said: it's not just your life at stake it's everyone on the road. X2. Quote KW T-680, POPEMOBILE Newmar X-Aire, VATICAN Lots of old motorcycles, Moto Guzzi Griso and Spyder F3 currently in the front row Young enough to play in the dirt as a retired farmer. contact me at rickeieio1@comcast.net Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.