'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

by relic7680 Sun Jul 02, 2023 9:02 am

I had the correct Delco Morraine # 5470409 master cylinder rebuilt for my car at White Post restorations in VA after having nothing but trouble with the new (overseas) and reman units. I sent the master back to them because the the lid leaked....and they machined the surface of the casting. The lid, bail handle, and gasket I have were purchased new as a set on eBay...includes the Dorman gasket that appears to be the only one available for this application. I have tried bending the handle to clamp down harder on the lid, as well as replacing the handle with a decent used GM one off a '72 Monte Carlo. I pulled a bit of fluid out of the rear chamber in case the level was too high and saturating the rear of the gasket.

NONE OF THIS HAS SOLVED THE PROBLEM. I'll put a few miles on the car and see a small stream of fluid out the back of the lid.

I can get an NOS # 18000779 lid on eBay, but the # 5470404 GM gasket is nowhere to be found.

HAS ANYBODY DEALT WITH THIS ISSUE BEFORE? Pics attached....thanks in advance.

'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET Img_7210

'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET Img_7212

'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET Img_7211

by 76Chevelle2Tone Sun Jul 02, 2023 11:12 pm

Well it has to be the lid assembly as it is not leaking from the master cylinder body. Where did you get the lid assembly? I have never had this problem myself so can't say what I did to fix. But I did buy a new complete cylinder assembly, rather than a cylinder one place and a lid another.

The only thing I can suggest is that you get the NOS lid and get another Dorman gasket and hope it is a lid misfit that is the problem.

by relic7680 Mon Jul 03, 2023 9:13 am

The aftermarket lid/handle/gasket assembly was purchased as a set on eBay, and is the only readily available setup. The handle in my pics here is a used OE piece off a '72 Monte Carlo.
I just purchased an NOS GM lid (no gasket or handle) off eBay yesterday that I will try soon.
The parts store masters I tried before (all leaked from the piston seal or would not bench bleed properly) had new lids and gaskets....probably should have swapped them with some of my old stuff to try...oh well.
One guy said that unless you have to pry the handle off the lid with a screwdriver that it's not tight enough....so I may revisit that with my aftermarket handle alterations.

by S3TPI Sun Jan 07, 2024 9:44 am

Many of the replacement GM master cylinders I have installed leaked from around the lid. Inspection of the lid, bail, and rubber showed nothing visibly wrong. In most cases, I found this leaking will stop after time, as the rubber seems to stick to the top of the reservoir.

by relic7680 Sun Jan 07, 2024 7:23 pm

What I ended up doing was sourcing an NOS GM lid gasket, and that solved the problem. It was hard to come by and I had to pay up....but that's what it takes these days.

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'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

'76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

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Re: '76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

Re: '76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

Re: '76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET

Re: '76 GMC Sprint LEAKING MASTER CYLINDER LID GASKET