Brake bleeding trouble-

[alaskajosh]

I've got spongy brake. A quick second stab firms them up. The problem is distal of the rear flex hose as I can clamp off that hose and get a nice hard pedal every time.

I've bled the snot out of them and for a little while after bleeding the pedal is hard (yes it lasts longer than pumping the pedal and compressing the air bubble lasts).. then after sitting overnight it's gone soft.

There must be some kind of leak to allow air in but in a positive pressure circuit (and a great deal of pressure at that) wouldn't any leak manifest itself as spilling/seeping fluid? Not sucking air as it sits? I see no such leaks.

Also, I've pulled apart the slave cylinders and by all appearances they're tip-top.

I'm beating my head here... Thank you very much for your time--

Josh

[Mercedesrover]

Maybe you need to adjust the brakes? Sounds like your double-pump is simply filling up the wheel cylinders to make up the distance of the shoes not being adjusted properly. Jack the truck up and adjust each cam until you just start to feel the shoes dragging.

jim

[friar mike]

2 things I can think of is to check is brake shoe ajustment and a leak in the master cyl inside between the front and back brakes there will be no leak outside of the cyl you see on this one. do you live in Alaska?

[Donnie]

I've got spongy brake. A quick second stab firms them up. The problem is distal of the rear flex hose as I can clamp off that hose and get a nice hard pedal every time.

I've bled the snot out of them and for a little while after bleeding the pedal is hard (yes it lasts longer than pumping the pedal and compressing the air bubble lasts).. then after sitting overnight it's gone soft.

There must be some kind of leak to allow air in but in a positive pressure circuit (and a great deal of pressure at that) wouldn't any leak manifest itself as spilling/seeping fluid? Not sucking air as it sits? I see no such leaks.

Also, I've pulled apart the slave cylinders and by all appearances they're tip-top.

I'm beating my head here... Thank you very much for your time--

Josh

I'm with Jim on this one, air can't get in : Adjust the brakes...............

[alaskajosh]

It'll do it even with the brake shoes jambed into the drums.

I'm banking on the friars diagnosis... ordered the MC rebuild kit today.

[alaskajosh]

So much for genuine parts-

I ordered the MC rebuild kit. The biggest cup washer/seal thing... the main one that keeps fluid from leaking out of the MC.. appears to be too big. It'll fit in there alright but so tight that the spring isn't strong enough to return the piston after it's been depressed. It just sticks in the cylinder anywhere you push it and is then very hard to knock back out.

The cylinder is in good shape and I lubed the tube and the washer with brake fluid so that it wouldn't have any excuse to stick.

Frustrating.

[friar mike]

Well there is something wrong here the seal should slide smooth no sticking it sounds like the wrong seal. back to the old problem if the brakes are adjusted up right or even jambed to take it out of the loop and there is no air in the system ( be very sure of it ) no leaks are found. check to make sure that you don't have a hose going ( I forgot that one earlyer) then it's in the master cyl. there is nothing else in the system. that as much as I can do here it would have to come in the my shop for me to see what I could find. Iam working on my SIII all day today trying to fix all that has been mucked up by a ( professional restorer ) it seem's I do alot of this kind of damage controll lately. well it won't be all fixed today more like all winter.

[alaskajosh]

-Brake pedal goes all the way to the floor on the first pump.

-Quick subsequent pumps will bring it up the pressure to some degree.

-The problem goes away completely (good pedal) when I clamp off the rear flex hose.

-I've taken off the drums and tightened ratchet straps around the shoes to ensure that none of the "give" is in the shoe travel.

-I've bled and bled removing the brake pipe mounts from the diff and bending the pipe down level and lower than the slave cylinders to prevent any stubborn air pockets trapped in a high spot. Tapping brake pipes while bleeding to encourage air bubbles to free up and travel.

-MC looked great in every way. I lightly honed it anyway and installed rebuild kit from RN.

-Slave cylinders look good inside and out.

-No external leaks of fluid anywhere to be seen.

I'm beside myself, gents. I'd take it to my local mechanics but I'm not sure what they're going to do that I haven't tried (and tried) and I'm not that wild about driving it in this state anyway.

Thank you in advance for any ideas of things I might try or look at.

Thanks, Josh

[BackInA88]

Have you replaced the rear flex hose yet?

[greenmeanie]

Just in case its the same thing from you desription you have a seal that is too tight.

If the master cyl has a bore of 5/8 and the overhaul kit then the main seal is often wrong and is for a 3/4" bore. It can be forced in there but it is WAY too tight. Yup, sometimes LR Genuine Parts are sh!t. They've been messing up 101 owners with this for years on clutch overhaul kit 601611.

The fix is to go to NAPA or equivalent and cross reference an overhaul kit equivalent to 8G8600L. It's also cheaper than the LR rubbish.

Cheers
Gregor

[alaskajosh]

Meanie: That seal had all the exact same numbers on it and was only 2 or 3 thousanths bigger (I don't remember the exact measurements). But it was too big... or the spring too weak??

At any rate.. the MC never leaked out from there anyway so I put the old seal back in. I'll use this new one to shim up a tippy barstool or something.

Backina': No, I hadn't. I probably should just to "rule it out" (that's wrenchicon for "throw more money at it"). What I am doing is clamping the hose just as close to the far end as I can so I can rule out as much of the hose as possible. When I clamp it off within 1/16" of the "T" I get a good pedal. Maybe the problem is in that last 1/16" but it's hard to imagine. It's a good idea though.. one I should probably do anyway just cuz'.

Thanks!

[4flattires]

Hose flex...

...can be a real source of pedal travel. You dont have to replace to diagnose this. Clamping off was a clue that Backina picked up on.

It sure sounds like you have ruled out shoe travel.

Have a helper observe if the hose expands when applying pressure. It only takes a little expansion to cause increased pedal travel. If possible, use a caliper to measure hose expansion (with and without pressure) to evaluate.

High performance applications take the braided s/s lines to avoid hose flex. On motorcycles, braided lines makes a real difference you can feel compared to OEM rubber hoses.

Jeff

[alaskajosh]

Well a member here, FriarMike, won a victory over this problem today in his shop. The answer was to jack the rear end of the car waaay up and then bleed.
All's well.
Thanks Friar!

[friar mike]

No problem well maybe a little had to take the round a bout method of getting it but it worked.