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Old 01-06-2020, 07:33 AM   #1
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R-12DC valve troubleshooting

I have a problem with the R-12DC relay valve in front of the rear axle. I've removed the valve twice and rebuild it with the Bendix service kit and it still malfunctions although it may not be the valve that is bad.

The problem is as follows:

1. Engine running, parking brake engaged, the system airs up to 115 psi pressure no problem. When the engine is shut off the system will hold that pressure as well.

2. Engine running, parking brake disengaged, the R-12DC valve starts to leak from the exhaust port and pressure starts to drop to about 70 psi. If parking brake is engaged again the R-12DC valve stops to leak.

3. If the engine is shut off with parking brake disengaged the R12-DC valve will leak through the exhaust port till the system pressure reaches 50 psi and then it will stop leaking.

Has anyone opened up this valve? Mine does not have a "differential spring" for controlling crack pressure but it could be that it is missing. The Bendix manual says that the differential spring is not required for most models but maybe on the TC1000 it is.
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Old 01-06-2020, 08:06 AM   #2
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replace it. I had the same vaslve leaking on my 1978 Superior... truck-Pro had it in stock.. i think it was like $30.. bolted it on and good to go..


-Christopher
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Old 01-06-2020, 09:29 AM   #3
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I don't think we are talking about the same valve. The R12-DC relay valve costs on average about $140. Besides that, there are several different springs for controlling the crack pressure and I would need to know which one to choose.

This is how it looks like.

http://triplertruckparts.com/r12-dc-...bendix-800481/
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Old 01-06-2020, 10:50 AM   #4
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Call a bluebird dealer for the right number and the crack pressure.

As far as the valve goes, are you sure it's an r12dc? Typically an r12dc is for service brakes and won't be affected by applying or releasing the parking brake.

The hoses that connect to it, where do they all go?
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Old 01-06-2020, 11:31 AM   #5
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pretty sure that it is.

There is a supply port hose and two delivery ports that go the ABS valves sitting right beside it. Then there are two control ports on top of the valve and another port that goes to a valve sitting between the two ABS valves righ below the R12DC valve.
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Old 01-06-2020, 01:19 PM   #6
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Okay, with it feeding the abs dump valves I'll assume that's what it is. What's the valve look like below the r12dc? It sounds like it might be the relay valve for the spring brakes, which would be an r14 of some variety.

Just for giggles. MAKE SURE THE WHEELS ARE CHOCKED, AND YOU FOLLOW ALL SAFETY PROCEDURES. Take the hose between the r12 and the r14 valve loose near the r12 valve, run your tests and see what happens. If air is leaking out of the hose, you found the issue.

On one really strange instance 10 or so years ago, I did have the diaphragm rupture in the anti-compounding valve on the spring brake valve. When that happened, air from the dash valve would back feed into the relay valve for the service brakes, which would cause rapid air loss out the relay valve exhaust for the service brakes.
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Old 01-06-2020, 01:28 PM   #7
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Also, I typically don't rebuild the valves. The rebuild kits usually aren't available, and I don't want to incur the responsibility of rebuilding it and it failing is the main reason. But if you're confident in your ability to rebuild them then have at it. However, the last time I shopped around for a reman vs rebuild kit, the prices were similar fwiw.
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Old 01-06-2020, 01:30 PM   #8
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so this isnt the valve that does the controlled apply of the spring brakes with the service brakes in the event the primary pressure goes flat but you still have secondary?
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Old 01-06-2020, 01:40 PM   #9
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Quote:
Originally Posted by cadillackid View Post
so this isnt the valve that does the controlled apply of the spring brakes with the service brakes in the event the primary pressure goes flat but you still have secondary?
I think you're confusing 2 valves.

The r12 valve is typically used for service brakes. The r12dc can be applied by both primary and secondary sides of the brake pedal valve(treadle valve), through it's double check valve. So if the primary side of the treadle valve fails, or the line is cut, etc. The secondary side of the treadle valve can be used in the relay valve to apply primary air pressure to the service brakes.

The r12dc is supplied solely by the primary side air tank. If you lose all primary side pressure, the relay valve will no longer function.

The R14 valve is usually used for spring brakes. It's the one where if you lose all pressure on the primary side, it will modulate down the pressure releasing the spring brakes to achieve a controlled braking function. Theoretically, it would no longer be the service brakes applied, but the spring brakes.

Clear as mud?
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Old 01-07-2020, 08:57 AM   #10
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This is what the valve looks like and how the Bluebird manual states its plumbed with the other valves. I haven't found any literature on a R12DC valve that has the all the connections of the one I have. I suspect it could be the R12DC I have is a substitute for the SR-7 valve in the diagram below + a standard R12DC valve. In other words that it substitutes the one valve for both of them.
Attached Images
File Type: jpg IMG_0112.jpg (218.5 KB, 11 views)
File Type: png Annotation 2020-01-07 074815.png (439.5 KB, 10 views)
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Old 01-07-2020, 09:50 AM   #11
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The r12 in your literature is just a standard r12 valve. What's on your bench is definitely an r12dc.

What does the other valve connected to the r12 look like?

Did you try taking the hose between the 2 loose?
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Old 01-07-2020, 10:51 AM   #12
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You wouldn't have an sr7 with an r12dc
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Old 01-07-2020, 07:15 PM   #13
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You have a brake chamber that leaking between the service brake half and the spring (parking) brake half. Your rear brake chamber has 2 lines going to each one. Remove the service brake line from each side. Build you air pressure, release the parking brake. Before air runs out, find out which chamber the air is leaking out. Replace that chamber. Most likely a 30/30 or 30/30 long stroke.
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Old 01-08-2020, 06:53 AM   #14
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Would a leaking brake chamber really cause the R12DC valve to leak?
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Old 01-08-2020, 08:15 AM   #15
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Usually if the chamber is leaking like that, the air will dump out of the exhaust on the abs modulator on the side with the bad chamber.
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Old 01-08-2020, 08:20 AM   #16
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Quote:
Originally Posted by Booyah45828 View Post
I think you're confusing 2 valves.

The r12 valve is typically used for service brakes. The r12dc can be applied by both primary and secondary sides of the brake pedal valve(treadle valve), through it's double check valve. So if the primary side of the treadle valve fails, or the line is cut, etc. The secondary side of the treadle valve can be used in the relay valve to apply primary air pressure to the service brakes.

The r12dc is supplied solely by the primary side air tank. If you lose all primary side pressure, the relay valve will no longer function.

The R14 valve is usually used for spring brakes. It's the one where if you lose all pressure on the primary side, it will modulate down the pressure releasing the spring brakes to achieve a controlled braking function. Theoretically, it would no longer be the service brakes applied, but the spring brakes.

Clear as mud?

makes total sense.. and you are right and it is an R14 valve that i replaced on my Superior.. it was leaking when the park brake was disengaged (system pressurized)..
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Old 01-08-2020, 08:23 AM   #17
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Simple to check though, just remove the hoses going from the modulator to the chambers and see if one is pushing air back into the modulator.

If there is air coming out of the exhaust of the relay valve. It's either bypassing the inlet/exhaust valve in the relay valve, or it's being backfed air pressure from one of the output hoses on the relay valve. Remove the hoses on the outlet of the relay valve and see if any are backfeeding air into it. If one is, then whatever component is connected to that hose is faulty. If none are, and you still have a leak. then the inlet/exhaust valve is faulty.
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Old 01-10-2020, 06:59 AM   #18
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Ok so I performed some tests. The valve that sits below the R12DC valve which feeds air into one of the delivery ports (not supply!) of the R12DC is a QR-1c quick release valve and it looks like this:


IMG_0116.jpg

IMG_0119.jpg

As per the Bendix service bulletin the operation of the valve with the spring brakes released is as follows:

http://www.plazafleetparts.com/uploa...1c_service.pdf


Quote:
When the spring brakes are released, air from the park control
valve flows through the QR-1C valve, causing the double
check valve and quick release diaphragms to flex and seal
the balance and exhaust ports. Air flows into the emergency
port of the spring brakes from the QR-1C valve delivery
ports.
Now, with the supply air connected to the R12DC valve and the delivery and control lines disconnected the R12DC valve will not leak. It will only leak through the exhaust port when I connect the line from the "balance port" of the QR-1C valve to delivery port of the R12DC valve. In other words there is constant air coming out of the balance port of the QR-1C valve when the spring brakes / parking brake is released. As per the above description when the spring brakes are released the balance and exhaust port of QR-1C (which is the one feeding the delivery port of R12DC) is supposed be sealed. The exhaust port does seal but the balance port does not. Looking at the diagram of the Q1RC valve I would guess the double check diaphragm between the supply port and the balance port is the one that is probably leaking.

Does that sound right?

Or could a brake chamber or something else be wrong?

Annotation 2020-01-10 080606.png
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Old 01-10-2020, 09:01 AM   #19
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Quote:
Originally Posted by Tomas View Post
Now, with the supply air connected to the R12DC valve and the delivery and control lines disconnected the R12DC valve will not leak. It will only leak through the exhaust port when I connect the line from the "balance port" of the QR-1C valve to delivery port of the R12DC valve. In other words there is constant air coming out of the balance port of the QR-1C valve when the spring brakes / parking brake is released. As per the above description when the spring brakes are released the balance and exhaust port of QR-1C (which is the one feeding the delivery port of R12DC) is supposed be sealed. The exhaust port does seal but the balance port does not. Looking at the diagram of the Q1RC valve I would guess the double check diaphragm between the supply port and the balance port is the one that is probably leaking.

Does that sound right?

Or could a brake chamber or something else be wrong?
Sounds like you found the issue.

There shouldn't be air flowing out of the control side of a qr1c valve. Whether the diaphragm ruptured, or whatever happened, that leak is backfeeding into the r12dc which reroutes the air into the open exhaust port of the valve.
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Old 01-18-2020, 03:12 PM   #20
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Had a very similar situation on a Air Braked trailer. Disassembled it and found corrosion. Cleaned it up and cured the air leaking issue
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