The Ford Barn

ursus · 03-05-2026, 01:33 PM

I need to replace the leaky 1939-41 Ford master cylinder in my coupe. I have a new and never installed Wagner brand master cylinder that came with a parts collection that I acquired back in the 1980's. The box was faded and a bit scuffed up then so I imagine the part could have been made back in the 1960's.

Is this old but new master cylinder OK to install as is or should I dismantle it to check the condition of the seals? I am concerned about the condition of a mechanism that has lain dormant for so long.

glennpm · 03-05-2026, 02:07 PM

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I'd do a visual inspection of the reservoir if it is rusty it either has had water inside from condensation or bad storage. Most replacement master cylinders, MCs, are pressure tested at the factory. I've found that most are wetted with brake fluid which is used as an assembly lube to protect the seals so look for evidence of that.

Add a little brake fluid to the reservoir and pump it a couple of times with the cover in place until you get fluid squirting out the port. Now block off the exit port on the master with a 7/16-24 bolt wrapped with a turn or two of Teflon tape. Thread engagement must stay clear of the female MC raised cone. This size assumes you have the stock 1/4" brake lines. You could also use a short stub of crimped off brake line. Add some new clean brake fluid to the reservoir and try to push in on the plunger. It shouldn't move but a tiny amount.

If you had a dual bowl MC, you would have to bench bleed to get rid of any air in the MC bore. This is not required for a single pot MC. Once bench bled, you can check it as above for a single pot MC.

03-05-2026, 01:33 PM	#1
ursus Senior Member Join Date: May 2010 Location: Oregon Posts: 1,534	New Old Master Cylinder I need to replace the leaky 1939-41 Ford master cylinder in my coupe. I have a new and never installed Wagner brand master cylinder that came with a parts collection that I acquired back in the 1980's. The box was faded and a bit scuffed up then so I imagine the part could have been made back in the 1960's. Is this old but new master cylinder OK to install as is or should I dismantle it to check the condition of the seals? I am concerned about the condition of a mechanism that has lain dormant for so long.

03-05-2026, 02:07 PM	#2
glennpm Senior Member Join Date: Sep 2014 Location: Mid-Coast Maine Posts: 3,346	Re: New Old Master Cylinder Sponsored Links (Register now to hide all advertisements) I'd do a visual inspection of the reservoir if it is rusty it either has had water inside from condensation or bad storage. Most replacement master cylinders, MCs, are pressure tested at the factory. I've found that most are wetted with brake fluid which is used as an assembly lube to protect the seals so look for evidence of that. Add a little brake fluid to the reservoir and pump it a couple of times with the cover in place until you get fluid squirting out the port. Now block off the exit port on the master with a 7/16-24 bolt wrapped with a turn or two of Teflon tape. Thread engagement must stay clear of the female MC raised cone. This size assumes you have the stock 1/4" brake lines. You could also use a short stub of crimped off brake line. Add some new clean brake fluid to the reservoir and try to push in on the plunger. It shouldn't move but a tiny amount. If you had a dual bowl MC, you would have to bench bleed to get rid of any air in the MC bore. This is not required for a single pot MC. Once bench bled, you can check it as above for a single pot MC. __________________ Archives of historical but relevant older articles: ------------- Hover mouse over the links below and click! ~~~~~~~~~~~~~~~~~--------------- Rumble Seat’s Notes Techno Source for the 1932 thru 1953 Flathead Ford Last edited by glennpm; 03-06-2026 at 10:50 AM.