air brakes use too much air

Now I ask again, which needle drops - one or both?


Neither the application gauge is going up.

 

I'm at a loss here, and need some clarification.

First the title directly suggests the brakes consume too much air(suggesting a leak), and the air tanks are dropping too much pressure. This is not the case? Rather the application gauge is rising too much? You should change the title of the thread if this is the case, it is very misleading and causing confusion.

In order to put air into the brake system, the tank pressure MUST drop. That's where the air comes from. Now you say sometimes hitting the brakes you have 15 psi on the application, sometimes 40-60, does the drop in tank pressure change? For example on the 15 psi application with trailer, your tank pressures might drop 5 psi. On the 40-60 psi application, what is the tank pressure drop? It should be more air out of the tanks. This is vital information, application pressure goes up, tank pressure goes down upon brake application.

 

I agree - the application pressure is a whole different story.

My first thought now is that contaminated brake shoes may be the culprit. An old timer also told me once that 'glazed' drums (from overheating) can also cause lack of brake effectiveness.

This is assuming that you're saying you require 40-60psi application to stop the truck.

If you're getting 40-60psi application with just light pressure on the pedal, then you have a defective treadle valve, IMO.

edit- you replaced the treadle valve, so now I'm out of options regarding the air system.

 

Another thing I've learned is the application pressure is *NOT* what is seen at the brake chambers. This is the pressure that the service side of the relays see. The pressure to the chambers is a different story due to various crack pressures and bobtail valves.

It sounds like the problem is in the foot valve. If you say the foot valve was replaced then I say the new foot valve is defective or was installed incorrectly.

However we still need the questions answered in post #23 to make a definitive answer.

Another question I have that hasn't been clarified here is does the application pressure change with the same foot pressure, or do you have to push harder and get more pressure to stop the same? Two different situations.

 

The service brake relay valve (hanging up) can cause this problem and be intermittent (not hanging up all the time). The valve can corrode from moisture to cause the piston to hang up. More signal pressure to the valve (which is what he is seeing on his guage) sometimes frees the valve to operate, sometimes not.

 

check the brake chambers again. some times you can hold the brake down and it will leak and sometimes it wont, because the plunger will seal up the tear in the diaphram.

 

He didn't ask how much the application pressure dropped. He asked how much the tank pressure dropped. If application pressure goes up, tank pressure goes down. You guys assumed incorrectly he was talking about application gauge dropping. He was not.

As far as the last sentence, he hasn't indicated either way. That's a question we still need answered in order to properly diagnose.

 

ummm no, we didn't asssume anything.

we know exactly what the op is getting at.

it's some other posters that are thinking opposite.

 

Windsmith asked how much does the pressure drop in the tanks, and is it the same amount. You and bubba both jumped on him saying no the pressure was going up. You both have the reading problem not us. The reason the question was asked by windsmith and I is because it is absolutely essential information. Three pages in and we still don't have the information we need.