View Full Version : Brake valve 101

Joe Camper
05-04-2018, 10:26 AM

Okay we're on the back wall of the steering Bay down at the very bottom of that bulkhead by the floor that's where this stuff came out of. The valve I replaced in the middle is a DC4 on the left that's your primary air the sending unit for the gauge is poking out the top and on the right is secondary air.

If you ever have brake tank pressure loss and both tanks are going down simultaneously together this is a good place to start because this is the only place that the primary and the secondary air is connected.

This DC4 what it does why it's there that valve floats and if the pressures become unequal in the primary and a secondary it always allows the higher pressure to supply the air coming out. The pressure in primary and secondary tank are always getting unequal when you step on the brakes because the secondary is supplying 2 axles in the primaries only supplying one.

After I stick this back in there there's another valve I'm going to pull it's called the sr1 and when I get it out I'll take a picture of it and try to explain it to you it's a pretty special valve that sr1.

Joe Camper
05-04-2018, 10:29 AM

Here is a different angle.

The dc-4 built back into its manifold with the primary AIR block on the left and the secondary air block on the right. Coming out the front of the air block is the sending unit for the electric warning light for low air pressure and coming out the top is the push in fittings going to the gauges on your dash.

Joe Camper
05-04-2018, 12:17 PM


Here is where the SR1 lives and what it looks like. It's a cousin to the DC4, both safety devices.

If u loose all service air pressure to the rear this valve comes on line using front brake tank air (till that runs out) and modulates air to the emergency half of the rear brake maxis and gives u a few applications to get woed up before the parking brake button pops and sets the emergency brake.

05-05-2018, 08:11 AM
Just great , Add a couple more complicated little valves into the mix for me to try and get my head around how and what they do. Your descriptions were pretty good on that, and thank you for going to the trouble with pics and all. But it brings up a couple of questions. When You said SAFETY I try to pay attention.

1) was there any indication that either the DC4 or the SR1 were going bad ? What are the symptoms ?

2) should these be changed out as a PM item , or only after failure ? Or might be a good idea after xx amount of years ?

3) Does the typical pre trip brake test tell us that these valves are ok ? From Your description of the SR1 it will send air to the primary from secondary in the event of a primary failure. With one system supplying air to two axles and one supplying air to the third axle. Is there a psi point like 50 or 60 or something where this thing kicks in that we can see the difference in our gauges equalize to verify that the SR1 is working properly ? Or would you basically have to create a failure to verify ?

That SR1 could make a huge difference in getting out of heavy traffic or off the road. Has mine ever been changed , do I need to keep one on hand , Heck I probably could not change one out if I had it anyway from the looks of all of your customized wrenches to get at that rascal :confused:

Thanks in advance Joe !

05-05-2018, 08:57 AM
The DC-4 is a double check valve or better defined as a shuttle valve. It's a very simple device that I now use for select chassis side air demands. I would not really expect it to be the cause of brake system pressure loss given it's function is actually to select air from the supply side with greatest pressure. Without the engine running it will go back and forth taking air from one supply and then the next IF there is a downstream demand for air. Let's say you have a leaking parking brake valve. In that case, with the engine off, the DC-4 will take air down almost equally on the primary and secondary air systems by design.

I suspect, and Joe will now best, the main failures that bring both primary and secondary air down at the same time are the SR-1 valve and parking brake valve.

Joe Camper
05-05-2018, 01:20 PM
You'll never know if these valves are bad or not unless you get into an emergency and you need them.

Bruce had me change these cuz his bus is allmost 30 years old now and he just wanted new ones there were no symptoms.

05-05-2018, 05:25 PM
The only way I know to test a DC-4 is open one supply line at a time. While doing so the output should always have an output and the open supply line should not leak. BTW, these are about $30.

Joe Camper
05-05-2018, 08:15 PM
My Peterbilts havent had them. They use an inversion valve. Less complicated.