Truck brakes won't lock

When you pull the knob it dumps the air from your brakes allowing the spring brakes to be engaged, I really do not recommend messing with the spring brakes if you do not know exactly what your doing. Check them to make sure the TEE bolts are not installed on the brake chambers. Past that if your release is not dumping the air in the system, might be time to have a mechanic take a look at it. There really is not too many pieces involved in the air release side of the parking park.

 

You checked your brake adjustment?

That would be the first thing to check when tires don't lock in place. Truck or trailer.

Slack adjusters are a common failure. And if the tires aren't locking. The brakes on the highway aren't working either.

 

Likely your quick release valve. The air release you hear should be short and sudden, dumping all air on the "out" side of the QR valve as soon as the brakes are set. That's where I would look first, especially if your brakes feel like they're operating normally otherwise.

You should have a 3/8" air line going to the QR valve. That is your signal line. I would remove that line immediately after setting the brakes.

If there is no air there, loosen one of the brake chamber supply lines. If there is still air there. R&R the QR valve

 

You don't feel. That don't tell you anything.

You inspect with your eyes.

Keep truck brakes released and look with your eyes. If you see gaps between the shoes and drums.

They're out of adjustment.

 

Good point. It's easier to just throw some more parts at it. No sense in spending the 10 minutes to check it out first....