Probelms with Master Cylinder Leaking

[donkdonk]

Hopefully someone can help me with something. Have a truck that was a forestry unit. No brakes, determined it to be airpack, changed it. Bleed. Brakes work. Next day, brakes no work. Put to pasture.

Fast Forward 6 months.

Bleed. Brakes! Next day, no brakes! Master cylinder leaking, change. Bleed. Brakes! 1 hour later, NO BRAKES! Master cylinder leaking!

Next Day, take master cylinder off truck with known good brakes. Install. Bleed! Brakes! 1 hour later, master cylinder leaking, NO BRAKES!

I have had 50 trucks or so, only one ever had a bad master cylinder. What are the odds that the next 3 were all bad?? What are the other variables that could cause the master cylinder to do this? We are using a power bleeder, but have used it on 20 trucks or more with no problems. Could this be causing this? Took apart 2 of them and the seals and cup look good.

They are all 3 leaking at the front seal... if you take the boot off fluid then just pours out.

What gives?

 

[FireFighterHill]

Can you see where it is leaking from? Is it losing fluid? if so the fluid has to be going somewhere.

 

[Floridianson]

What gives = bad luck

 

[donkdonk]

What gives = bad luck

It is hard for me to wrap my head around that... 1 in 50, now 3 in a row. Got to be over a million to 1 chance on that. Not to mention the last one came out of a truck with known good brakes, no leaks.

 

[gringeltaube]

You say seals and cup looked good.... and how was the rest, I mean the cylinder surface, especially where the outer seal lip neads to seal? It seems you are swapping around old used parts, so don't expect to much....no leaks doesn't mean much to me...

Also, is the pedal free travel or pushrod lenght adjusted right on the problem truck, to allow the piston to come back all the way to rest against the snap ring?

G.

 

[poppop]

You need to disassemble the bad ones and have a look at the bores. I have had good luck honing to clean up and using old cups. I do usually install new ones though. I have cleaned up three airpacs in the last year and used the old parts and they are working great. Sometimes just removing a part and reinstalling it is enough to make them go bad. I think that is where the old saying comes from that says if it ain't broke don't fix it.

 

[donkdonk]

You say seals and cup looked good.... and how was the rest, I mean the cylinder surface, especially where the outer seal lip neads to seal? It seems you are swapping around old used parts, so don't expect to much....no leaks doesn't mean much to me...

Also, is the pedal free travel or pushrod lenght adjusted right on the problem truck, to allow the piston to come back all the way to rest against the snap ring?

G.

Well we took apart the first one that was leaking, and the seals looked good but there was a little pitting on the cylinder surface where the seal sits, which I would guess is why people hone them when they rebuild them, after we tried the 2nd one and it leaked, we took it apart and it looked even better... some very light pitting. The 3rd one (out of a running, driving, stopping with no leaks truck) is the one in it now, and it is leaking as well.

After bleeding the brakes: Pedal comes all the way up, you can wiggle the linkage so it is not in a bind or anything, the pedal looks and feels perfect, but after 5 pumps you start to lose pressure, a few more pumps and the pedal stays on the floor. Then you look under the truck and the master cylinder is leaking.

Thanks for the help.

I am going to try rebuilding one tomorrow, but honestly I don't know how all 3 of these could be bad, but we will see I guess.

 

[poppop]

Strange things happen, you just have to keep walking down the path until the light comes on and you figure out what the problem is. I have walked the path many times and spent hours thinking about problems and so far have been successfull in the end. It gives one a warm feeling which is great because until the light comes on it is very furstating.

 

[runk]

Is the pedal travel adjusted in the normal range ? I've seen this kind of problem on the old British cars I play with. Take a well used, but working fine no leaks master and swap it into another car, instant leaks. Usually happens because only part of the cylinder is used, a different adjustment uses a different part of the cylinder (same stroke length, just in a different part of the cylinder), and that area of the cylinder is pitted or rusty, and tears up the old seals.

 

[gimpyrobb]

I have a brand new in the box master cyl for $50 if you want it.

 

[woodywood]

if you look real close the fluid is coming out of the top of the master cylinder your airpak is stuck letting air backwash through the master cylinder!the reason i know this is cause mine did the same thing airpak rebuild problem fixed!

 

[gringeltaube]

if you look real close the fluid is coming out of the top of the master cylinder............................................................

..........and then somehow getting inside the boot...??????

....better look real close at the end of post#1......: "They are all 3 leaking at the front seal... if you take the boot off fluid then just pours out."

Sorry,

G.

 

[donkdonk]

Well, a 4th one didn't leak, but then the front 2 wheel cylinders leaked, changed those out and you get a good pedal, but it's just not right. You can drive it around the block, when you get back you HAVE brakes, but just not much. Can't see any leaking fluid anywhere.

 

[FireFighterHill]

Sounds like air in the system.