M809 Wrecker Brakes Locking up

[KTanis]

Just picked up and M809 5-Ton Wrecker, Having trouble with the brakes. When you press the brakes pedal goes very low and the brakes don't work at all, but after pressing the pedal a few times the brakes lock up and won't release until you really stand on the pedal and then they release. The master cylinder was empty when i got it, but i don't see any leaks, i did top it off. Is it possible it has air in the system now from being empty? Where should i look first?

 

[NDT]

For sure you will need to rebleed the brakes to remove air from the system. Hopefully you did not contaminate the system with DOT 3 fluid, you must use DOT 5. Not releasing can be due to the pedal or piston in the master cylinder not fully retracting.

 

[Ajax MD]

The brakes locking up is sometimes caused by a faulty air pack.

The brake fluid didn't just disappear, there has to be a leak somewhere. If the reservoir was truly empty, you'll need to bleed the system. There's bound to be air in it somewhere. Do a search for "power bleeder" and you'll find descriptions of how to make a cheap, effective tool to help you bleed the whole system.

I'd start by bleeding air from the system, check for leaks and then move on to the air pack, if necessary.

 

[VPed]

Also check to make sure the brake master cylinder piston is retracting all the way when the peddle is released and the right front glad hand has to be vented.

 

[fasttruck]

Micro brake part of the mix ? wreckers have a micro brake to hold the brake on as you sat in the cab and held the brake pedal down. These can fail to release also, Operated by a push button on the dash.

 

[SCSG-G4]

Actually it is a MICO brake part, not 'micro' (TM is wrong). It acts as a one way valve when setting up the wrecker to be stationary. Operator error in pushing the brakes real hard and hitting the switch. Problem there came when it was time to release the brakes, as it took MORE pressure on the MC side to release the rear brakes. Temp fix was opening a bleed valve on one of the wheels slightly, permanent fix was educating the driver. If you do have a problem with the MICO valve, it's a big problem as that part is no longer made, and to get at it, it is easier to take the entire wrecker bed off and go in from above. I hope your problem is with the air pack or the MC.