Master Cylinder, brake fluid geyser

Hello all, did a quick search and didn't find anything like the issue I am having. So here it goes. I got a 1971 AM General 2.5 ton. When purchased didn't have any brakes. Went through the whole system, replaced the Master Cylinder, Air Pack, one rear brake line, all wheel cylinders, three sets of pads and of course Dot 5 fluid. After doing a major and minor adjustment on the system it was time for fluid.

First time bleeding the system we did it the old fashion way, one person pump and add fluid, the other person bleed. With no pressurized air in the air system, we did went about doing our 7-point bleeding.

When we got toward the front of the system fluid started shooting up out of the master cylinder fill cap to the point of hitting me in the face when the returning the brake peddle to the upper most position. Soon after we ran out of the gallon of fluid I purchased and ordered another.

Second attempt I used the pressure sprayer method with the adapter from Big Mikes. Went through the system and blead the rest of the junk and air out of the system. Started the truck, ran it up to temp. and drove it 25 yards and hit the brakes. The truck stopped. I did a quick walk around and found a puddle under the master cylinder. After investigating I found the fluid leaking out the top of the Master Cylinder where it connects to the vent line. When I hit the brakes again it went right to the floor. No brakes.

Why would that much pressure be coming back into the master cylinder both with and without pressurized air in the tanks? Only thing I can think is that something is off with the new old atock long style Airpack. Anyone else ever heard of this type of issue. Big Mike and I are both stumped.

Thanks in advance

Did you adjust the new shoes? I can see this happening if all the cylinders have to retract a huge distance.

Try the pedal without having air pressure. Does it still spray?
Unhook the vent line, get air pressure and try again? Does it spray now?
Pull the boot back at the front of the master cylinder. Does fluid come out?

cattlerepairman said:

Try the pedal without having air pressure. Does it still spray?
Unhook the vent line, get air pressure and try again? Does it spray now?
Pull the boot back at the front of the master cylinder. Does fluid come out?

Click to expand...

They did it w/o air pressure when they were bleeding the brakes the first time.

NDT said:

Did you adjust the new shoes? I can see this happening if all the cylinders have to retract a huge distance.

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Yes, both Major and Minor adjustments per the TM completed.

cattlerepairman said:

Try the pedal without having air pressure. Does it still spray?
Unhook the vent line, get air pressure and try again? Does it spray now?
Pull the boot back at the front of the master cylinder. Does fluid come out?

Click to expand...

Yes but only when letting off the pedal.
Comes out the top yes.
I will check

Here are three threads that should give you more ideas:

-the air pack can blow air through the vent line and cause what you describe; a leaking cup/seal or incorrectly assembled air pack can cause this
- there is a link to other parts of the brake system, i.e. non-functional trailer glad hands that can cause what you describe

I read your first post again and can’t tell if the problem happens only when the air system has air or anytime. The links posted above are good tech to follow if it happens when there is air in the airpack system.

If it happens when there is 0 psi in the air system. One suggestion is to adjust your brake shoes. If they are not close to the drums, the spring pulling them back will push fluid back through the master cylinder. The further they travel back, the more fluid gets pushed. Here is a video about the “Major” and “Minor” shoe adjustment:

Barrman said:

If it happens when there is 0 psi in the air system. One suggestion is to adjust your brake shoes.

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See his post #5 .........

Thanks. I thought I had read that the other day but just looked back at the first post.

I have a sneaky suspicion that the adjustment is too far off. Shoes need to be nearly dragging.

New or rebuilt air pack? The reason for asking is that if I remember, there was an issue in the rebuild instructions in one version of the TM. It caused an issue, don't remember what it was. AND, if it was/is a new old stock air pack, things could be dried out, moisture could have got into things and made rust rings where parts sat, etc.

It sounds like it didn't happen until "after" you did the rear axles and were moving on to front ones?
If thats the case I would back up and do the rears again just to be positive they're correct. Remember that the two cams on one wheel turn in opposite directions.

NDT said:

I have a sneaky suspicion that the adjustment is too far off. Shoes need to be nearly dragging.

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Yup, that's how they were adjusted.

LowTech said:

It sounds like it didn't happen until "after" you did the rear axles and were moving on to front ones?
If thats the case I would back up and do the rears again just to be positive they're correct. Remember that the two cams on one wheel turn in opposite directions.

Click to expand...

Well yes, but the system didn't have any fluid in it to begin with.

WillWagner said:

New or rebuilt air pack? The reason for asking is that if I remember, there was an issue in the rebuild instructions in one version of the TM. It caused an issue, don't remember what it was. AND, if it was/is a new old stock air pack, things could be dried out, moisture could have got into things and made rust rings where parts sat, etc.

Click to expand...

NOS, bought from Big Mike

Barrman said:

I read your first post again and can’t tell if the problem happens only when the air system has air or anytime. The links posted above are good tech to follow if it happens when there is air in the airpack system.

If it happens when there is 0 psi in the air system. One suggestion is to adjust your brake shoes. If they are not close to the drums, the spring pulling them back will push fluid back through the master cylinder. The further they travel back, the more fluid gets pushed. Here is a video about the “Major” and “Minor” shoe adjustment:

Click to expand...

Yup, that is how I adjusted mine. It happens with both air in the system and no air in the system.

So master cylinder rebuild fixed the geyser problem. Now I have a leak somewhere else int he system where air is getting in. So ya, fun fun.