Brakes come & go

So I been working on the brakes since I got the truck & I THOUGHT I had it fixed then i got a surprise. I put a new master cylinder & air pack on & bled the whole system. I ran about a half gallon of brake fluid through it w/my power bleeder & it seemed good to go. The pedal felt a little soft but was consistent, stoped a few inches from the floor & stoped the truck fine. That day i drove it maybe 2mi & all was well. Came home checked for leaks & called it a day. About 5 days later I leak checked it again & went for a ride. For the 1st 20min or so the brakes were great. Pedal was higher & more firm than the 1st drive then it went to the floor. It pumped up & seemed to be ok for 5-6 more stops then it went to the floor again & then after several pumps it was super hard, I mean I had to stand on it w/everything I had to stop it. I eased into a gas station & hoped out & took a quick look under the truck & again saw no leaks. Sat idle for maybe 10-15min & eased home. On the way home the pedal felt pretty mushy. Im at a loss. I know the next step is to pull the drums & check the wheel cylinders but I really don't see a blown wheel cylinder coming back like that. I duno. What yall think?

Did you bleed the airpack first then go to the farthest wheel and work back toward the airpack?

But it probably would be wise to just rebuild all the wheel cylinders so you know what you got.

If the pedal was super hard and provided little braking you lost air pressure assist. Check your gladhand (service/trailer brake) connection valves. I just had the same experience on my M275 and I had left the valve open on accident after towing a trailer. If all are closed then monitor your air pressure while your symptoms occur to help diagnose wether you are losing all air pressure or your air pack is malfunctioning.


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Yes I bled the air pack 1st then went to rear passenger wheel & worked my way to the front drivers wheel. I also let the fluid flow well after i saw the last bubbles. I plan on replacing all the wheel cylinders & brake hoses just haven't got there yet.

When my pedal got super hard i looked at my air gauge & it was at 90+. I will check my glad hand. If its open it's been open this whole time cause i haven't touched since i got the truck.

One other tip. The valves handles are opposite than standard orientation. Parallel with the valve is closed and perpendicular is open. At least has been my experience so far. Very confusing when your used to the opposite.


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AZORRAT said:

One other tip. The valves handles are opposite than standard orientation. Parallel with the valve is closed and perpendicular is open. At least has been my experience so far. Very confusing when your used to the opposite.


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This varies. I've seen them both ways. Sometimes they're labeled.

Back to the brake symptoms - I've experienced similar except for the hard pedal. Are you losing fluid out the vent under the hood?

I was before i changed my air pack but not now. At least I don't think so. I'll have to clean the area below the slobber tube & inspect it real good. With the old air pack it was super obvious, brake fluid dripping on the ground.

Is the air pack new or rebuilt?

New. Its supposed to be actually new & not new old stock. I got it from Sarafan in FL. The guy said they got it straight from the manufacturer aprox a yr ago. It looks new.

Sounds like the airpack is bad. Had the same problem happen right after I put in a rebuilt one on a truck.

Thats kinda what i was thinkning I was just hoping I was wrong. I just spent $425 on this thing. I sure hope they will exchange it.

Not assuming ANYTHING but this sounds VERY similar to what I am chasing. Excellent pedal while bleeding. Sits ( a couple days ) and pedal disappears and fluid disappears from m/c with no sign of leaks anywhere. I have been told the problem is the air-pac. Internally fluid leaks into the air side while sitting without air pressure and air leaks into fluid side when air system is charged ( engine running ). I trust the source that provided the intel and have done some other research. This is seeming to be a common thread with this air over hydraulic set-up. If the air-pac isn't PERFECT it will create ALL manner of problems.

I will NEVER advocate anyone else "modifying" their brake system on my advice but MY next move is to by-pass the booster with brake line from the m/c direct to the tee going to the front and rear brake tubing. This will make the brakes old school MANUAL (no boost). Which I personally am fine with. I know what the brakes on these Rockwell axles can stop without boost when working properly. If I have good luck I will share my results.

Yeah, I've only had the truck a month or two & i already hate this brake system. Hard to believe the military used this system for so long.

I wonder if the pedal would be as hard with no air pack like your talking bout as it is w/a bad air pack. I've had trucks w/manual brakes before but what I experienced the other was way stiffer than normal manual brakes.

My suspicion would be that the piston inside the pac is in a position that is actually stopping the fluid from going to the w/c s ( as yet unproven to myself ). When mine are unboosted they feel just like old fashioned manuals, 1/2 of free travel then stiff with maybe another 1/4 - 1/2 inch when I stand on it. I believe you know what I mean. Another fella in the Deuce thread is fighting with his too.

Yeah Yeah,
This is all really nice information.
So get a hundred or so bucks and a lot of gumption and whereititis and get under there and replace, replace with new, every rubber brake hose under the truck. Then go back to what you were doing before with all the steel and iron thingies.

Have you adjusted the brakes yet? If there is too much gap between the pad and the drum you can have these issues. Also have you been in the hubs yet? We had a truck where everything was replaced and still no brakes. Turns out one of the brake shoe pivot bolts came out. I'd check the easy stuff first, might keep you from spending money on parts that arent broken.

No, I haven't had the wheels off yet. That's going to happen this weekend hopefully. I really hope I find something simple once I do pull the wheels/drums.

Kinda hard to get the brakes working till you see what your working with. Why waste all that fluid when you haven't even checked to see if the wheel cyls work?

Well I had to buy a 3/4 impact & a lug socket & a set of big jack stands....... The master cyl was dry & all kinda rusty/crusty looking & a new one was only $65 then the air pack was blowing fluid outa the slobber tube so that got replaced. Then i fig why not bleed it & see what if feels like & it felt decent, at 1st.