Master cylinder blowback

[73440Roadrunner]

Hey I'm looking for some help. I have master cylinder blowback on the front reservoir (rear brakes) on 73 roadrunner with of powerbrakes. It started as a drip but within a few days has gotten worse.
I assume probably air in the lines so I'm in the process of replacing the rear brake cylinders, replacing the fluid, bleeding to hope resolve this. Just wondering if it could be something else that I'm missing that could be the cause. Thanks in advance fir your help

 

[vance.dykes]

I don't understand your terminology of blowback from a master cylinder if you're talking about when you have the cap off and you hit the brakes and you see fluid shoot up that is normal can you help describe more of what you're saying

 

[PurpleBeeper]

Yes, what is “blow back”. As mentioned, if the cap is off, that’s normal (rinse off your paint ASAP). If it’s leaking with the cap on, replace the rubber diaphragm in the cap

 

[73440Roadrunner]

I don't understand your terminology of blowback from a master cylinder if you're talking about when you have the cap off and you hit the brakes and you see fluid shoot up that is normal can you help describe more of what you're saying

Yes that exactly is what's happening except it blows out from under the cover
The gasket looks good and its held pretty tight with the bar over the cover

 

[493 Mike]

Yes that exactly is what's happening except it blows out from under the cover
The gasket looks good and its held pretty tight with the bar over the cover

Still has to be the gasket leaking.
Mike

 

[73440Roadrunner]

Still has to be the gasket leaking.
Mike

Ok thanks.. I'll find a replacement and give it a try

 

[dadsbee]

Ed had an NOS a couple years back... possible still does!

FOR SALE - N.O.S. 1969/71 Master Cylinder Gasket

 

[YY1]

Do the brakes work?

Do they work at all four corners?

There is a potential issue where the cylinder in the MC bore can bypass the line and recycle the fluid into the reservoir.

That shouldn't be a high enough pressure to cause leakage.....

Remedy is rebuild of replace.

 

[vance.dykes]

Yes that exactly is what's happening except it blows out from under the cover
The gasket looks good and its held pretty tight with the bar over the cover

You're going to need to clean the surface of the master cylinder area and replace the rubber with a new one over time some of these newer rubbers or maybe yours is old and has saturated the rubber and it's not Sealing as it should Dorman makes quite a bit of the replacement covers in their helpline

 

[vance.dykes]

You could even use a straight edge and a feeler gauge to make sure it's not overly warped from side to side and front to back on the master cylinder ceiling surface to the cap I've had to file them before to make sure they're flat put grease on your file this way you can clean the grease with brake clean

 

[YY1]

How much leakage is the OP getting?

I've never had one that completely seals 100%.

That was a big factor in my conversion to DOT 5 fluid.

 

[CHGRSE74]

If your brakes are working and stopping fine just replace the rubber cap seal. If yours is an OG cylinder, good luck… almost impossible to find. Mine was seeping even after reshaping the bale for more tension. My cylinder was also leaking out the back and wanted to get the original numbers correct one. Nothing available so I went with a Chineseium one that is somewhat similar. At least I can still drive it.

 

[73440Roadrunner]

How much leakage is the OP getting?

I've never had one that completely seals 100%.

That was a big factor in my conversion to DOT 5 fluid.

About 1/2 the small side of the reservoir with 10 depression of the pedal so a lot of fluid

 

[73440Roadrunner]

If your brakes are working and stopping fine just replace the rubber cap seal. If yours is an OG cylinder, good luck… almost impossible to find. Mine was seeping even after reshaping the bale for more tension. My cylinder was also leaking out the back and wanted to get the original numbers correct one. Nothing available so I went with a Chineseium one that is somewhat similar. At least I can still drive it.

I'm looking for one but nothing local so I'm going to have to order one. Its a dorman #42076 or I think oem #2944227 (small side on the curved side of the reservoir)

 

[YY1]

About 1/2 the small side of the reservoir with 10 depression of the pedal so a lot of fluid

Either you've got a severely bent lid, debris (rust?) on the lip, severe warp, plugged line, or the piston seal bypass issue.

That's all I can think of.

 

[MWbirdLOVER70]

Still has to be the gasket leaking.
Mike

Or the area where the rubber lays on the master housing is damaged or crumbling away

 

[dvw]

Those style masters seals are prone to seep even when new. That's what sent me down the Silicone brake fluid trail over 30 years ago. The modern aluminum masters seal much better. If you have nice paint, go silicone.
Doug

 

[YY1]

How much leakage is the OP getting?

I've never had one that completely seals 100%.

That was a big factor in my conversion to DOT 5 fluid.

That's what I said.

But the OP says ten pumps.

That seems more severe than a "weep".

 

[Macdon221]

take an fine diamond file...one that is on one of those hard plastic holders....its straight.....remove the cover and seal....clean the top edge of the master cylinder.....run the file across the top a few times and you can see right away if the top edge of the master is flat or not. You can see where the file is taking on the top edge. If it is not straight...it will leak.

 

[73440Roadrunner]

Hey I'm looking for some help. I have master cylinder blowback on the front reservoir (rear brakes) on 73 roadrunner with of powerbrakes. It started as a drip but within a few days has gotten worse.
I assume probably air in the lines so I'm in the process of replacing the rear brake cylinders, replacing the fluid, bleeding to hope resolve this. Just wondering if it could be something else that I'm missing that could be the cause. Thanks in advance fir your help

Just wanted to update my post. After replacing the rear brake pistons I started the bleeding on the right side. With 25psi vacuum no fluid was being drawn thru so I checked the left side same no fluid. So I cracked the bleeder on the right side and the vacuum dropped so it was drawing air across the rear axle lines. Next I disconnect the line at the proportioning valve and pumped up the vacuum and it held so I disconnected that line from the short rubber hose at the rear axle and fluid drained out of the metal line. So it appears the blockage is the short rubber hose that connects the metal line from the proportional valve with a splitter that feeds 2 metal lines one to the right and one to the left drums. After I replace the rubber hose I'll see if that is what was causing pressure to build and push the fluid out of the master cylinder. I'll update if that resolved the issue once its all put back together (waiting on parts).
Thanks for all the suggestions and support tracking down this problem