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M923 Brake Leak

Elk1111

Elk1111

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I just bought a 1984 M923 that runs and drives great but I have an substantial air leak when the brakes are pressed. The air is escaping into the intake. I have read the TMs and hope to start pressure testing it in order this weekend. My question is what kind of pressure gauge would work best for these tests.
I am assuming I have a bad diaphragm or two.
 
simp5782

simp5782

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Several active threads the past month on this. Everything from treadle valves to relay valves. Simple way to do this is to figure out which tank is leaking down and attack the appropriate system that way.

If the secondary side is dropping pressure start at the Q2 valve in front of the steer axle, remove the vent line from there and see if it dumps lots of air when the pedal is depressed.

Primary side is leaking we start at the vent block on the frame and work the way thru the system.
 
Elk1111

Elk1111

Well-known member
282
490
63
Location
Las Cruces NM
simp5782 said:
Several active threads the past month on this. Everything from treadle valves to relay valves. Simple way to do this is to figure out which tank is leaking down and attack the appropriate system that way.

If the secondary side is dropping pressure start at the Q2 valve in front of the steer axle, remove the vent line from there and see if it dumps lots of air when the pedal is depressed.

Primary side is leaking we start at the vent block on the frame and work the way thru the system.
Click to expand...
Are there any gauges that work better than others for these tests. I’d prefer to buy the correct one the first time verses wasting money on the wrong tools for the job?
 
simp5782

simp5782

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No tools needed. You have air leaking to the intake stack. Only certain air valves on the truck vent there. Q2 Valve, front and rear relay valves over the axles, brake pedal valve, transfer case interlock solenoid, R4 valve in the step.

Basically the lines going into the block in the frame give a lead to which component is failing and where the air leak is coming from.

Basically a 5/8, 13/16,7/8, and 15/16 wrench would be needed
 
Elk1111

Elk1111

Well-known member
282
490
63
Location
Las Cruces NM
simp5782 said:
No tools needed. You have air leaking to the intake stack. Only certain air valves on the truck vent there. Q2 Valve, front and rear relay valves over the axles, brake pedal valve, transfer case interlock solenoid, R4 valve in the step.

Basically the lines going into the block in the frame give a lead to which component is failing and where the air leak is coming from.

Basically a 5/8, 13/16,7/8, and 15/16 wrench would be needed
Click to expand...
I’ve traced the leak back to the QR valve in the front of the truck. Is there a way to clean/rebuild that or is it best to just replace it?
 
simp5782

simp5782

Feo, Fuerte y Formal
Supporting Vendor
12,131
9,410
113
Location
Mason, TN
Elk1111 said:
The new Q2 valve is not threaded on the bottom down facing port like my old one. Is there a different way to connect this or did I just get the wrong part?
Click to expand...
Post a pic of what you have? The new part i mean

@fuzzytoaster had an issue awhile back getting the correct one, due to its design I can't find his old thread though
 
Elk1111

Elk1111

Well-known member
282
490
63
Location
Las Cruces NM
simp5782 said:
Post a pic of what you have? The new part i mean

@fuzzytoaster had an issue awhile back getting the correct one, due to its design I can't find his old thread though
Click to expand...
I don’t have a picture right now but the new one is exactly the same except the bottom exhaust port is not threaded. The tee where the two bottom hoses connect will not be able to be attached to the new one.
 

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