Air brakes slow to release and squealing

[Sunbear]

Hi my airbrakes stick and squeal when ive had to start up and move the.bus immediately like when trying to avoid a street sweeper ticket. It happens when im at 120 psi is this the sign of up coming trouble? It's a scary feeling but once she is warmed up they release just fine. I figure being able to stop is more important than being able to go,especially because my road dog is my 4 year old son

[Brad_SwiftFur]

I am a driver by profession and I have a tanker trailer that does this - it has a slow air leak and the air tanks have to recharge before the brakes will release, typically at around 60-80 PSI (in the trailer). If your air tanks are at 120 and the brakes are still slow to release (more than 2-3 seconds), you may have a restricted air line to the parking brakes. Many trailers have a "brake control valve" that can do this as they age; not sure if buses have these or not. The service brake has a separate air line so stopping should not be affected if this is the problem. I would also make sure the air tanks are drained of moisture. Air systems typically have a "wet" tank and 1 or 2 "dry" tanks, all of which should be purged periodically, especially in cold weather. The "wet" tank should be purged every time it is driven (many of them have automatic valves that do this), for daily drivers, it should be part of a daily routine, usually done when shutting down in the evening.

[Robin97396]

If you hear the street sweeper coming it's already to late. A car, yes. A bus just takes more time to move.

[cowlitzcoach]

I am not clear as to when your brakes are slow to release and squeal.

Having spring brakes that are slow to release is not uncommon, particularly if you live in an area of high humidity or lots of rain. It doesn't take much to get the brake lining to get stuck with rust onto the drums.

Even when the brakes are not rusted onto the drums it takes a few counts before the spring brakes release completely--between a 5-count and a 10-count is not unusual.

If your spring brakes release within the 10-count I wouldn't worry about them. I would be more concerned about what pressure the spring brakes apply automatically--too high and you won't get enough warning to get out of the traveled lane of travel and too low and you will have no service brakes left at all.

When the drums are rusty the brakes will grab and squeal quite a bit until they are all shined up again.

Some brake linings actually squeal louder and longer than some others.

I remember one set of linings I had on a bus that if I applied just the right amount of pedal pressure I could get the brakes to squeal all the way down the hill. Just a little more or a little less pedal pressure and no squeal. If the neighbors on the downgrade weren't awake before I headed down the hill I doubt they were after.

[Sunbear]

Thank you again for your help

[Brad_SwiftFur]

Seconding Cowlitz's comment ... the parking brake should "pop out" by itself at a certain pressure, typically between 60-80 psi (60 being the most common). To test this, stop someplace level and safe, leave the brake "released" (usually pushed in, but on a few older models it may need to be pulled out) ... with the engine off, push the brake pedal a number of times to deplete the air pressure. If it drops below 60 before the brakes apply, that's cause for failed inspection. ... OK, so if it pops at 58 or 59 PSI no one's likely to make an issue of it, but still released at 40? Yeah, that valve needs replaced. Fortunately these things tend to be fairly durable and reliable, the usual reason for replacement isn't always the pressure at which it pops, but usually because they begin leaking air.

[2kool4skool]

Max pressure 120 psi.

Buzzer and light at 60 psi.

Spring (parking) brake should activate at 40-20 psi.

[Brad_SwiftFur]

The one in my Volvo (road tractor) pops at about 60; I can get it to release around 40 or so if I need to move it without running the air up any more than necessary. Warning alert is 60, air systems (air suspension, etc) charge around 80. This truck has a slow air leak (or probably a lot of 'em), but not enough to fail the air-loss test.