Master Cylinder Rebuild

[65_BRG_Sprite]

'65 dual master cylinder rebuild. Trying to install the 'pistons', having trouble getting the rubber cups into the bore. Very leery of using tiny dull screwdriver to gently introduce the rim of the cup into the bore. Cups are generously lubricated with brake fluid. Suggestions please!

 

[Bayless]

Not really an answer for you but those things are notoriously tricky to do. The cylinder mouth usually has at lease a minor taper to help. getting the cups started. I have had some luck starting it very slightly cocked then twisting while pushing to coax it in.

 

[65_BRG_Sprite]

Thank you. This is a Moss replication piece and a Moss rebuild kit, the catalog numbers are correct. The cups seem way too large compared to what was removed. Hopefully tomorrow I'll drag my pudgy self up into the attic and get the original MC down for comparison. Something is not right.

 

[BobHaskell]

Aren't there two master cylinders - with different bore diameters?

 

[65_BRG_Sprite]

Yes there are two sizes; the 948 had its own 7/8", and later cars w/disc brakes use a 3/4" bore. I pulled the original MC from storage and measured 3/4". I compared the new cup sizes to two different old sets of pisons/cups and the new ones are clearly larger. The package doesn't have a "Moss" logo (there is no manufacturer name/logo) so I can't blame Moss. The catalog numbers are correct. Perhaps earlier cups inserted by error. Regardless I can't afford $25.00 losses.

 

[65_BRG_Sprite]

72 years and this green horror have taught me several things. One is if it is too hard I'm probably doing it wrong. So said I go out to work on the MC when I'm in the right mood and cautious about strong-arming it. Today was a revelation. Again, working carefully, tried to work the cups into the bores. Will not fit.. Looked at an old MC's push rods and voila- the diameters are different, the ones on the new MC are visibly larger, therefore when the cup is fitted they are stretched too large to fit the bore. I don't own a micrometer to measure the difference but it's visible. Gently removed the cups, installed on the old push rods and they are a go! Push rod lengths are the same. Happily I pushed the assemblies into the MC and started to install the end cover plate when it broke in half. Key back to the start of this paragraph- NOT strong-arming it! Back to the old model and reused the end plate, Never again will I try to save a few bucks on a cheap replacement part. It's cheap for a reason. Perhaps tomorrow I'll bench test it, or perhaps drink heavily. Saving up for a new MC from reliable name brand. TRW folks?

 

[JPSmit]

72 years and this green horror have taught me several things. One is if it is too hard I'm probably doing it wrong. So said I go out to work on the MC when I'm in the right mood and cautious about strong-arming it. Today was a revelation. Again, working carefully, tried to work the cups into the bores. Will not fit.. Looked at an old MC's push rods and voila- the diameters are different, the ones on the new MC are visibly larger, therefore when the cup is fitted they are stretched too large to fit the bore. I don't own a micrometer to measure the difference but it's visible. Gently removed the cups, installed on the old push rods and they are a go! Push rod lengths are the same. Happily I pushed the assemblies into the MC and started to install the end cover plate when it broke in half. Key back to the start of this paragraph- NOT strong-arming it! Back to the old model and reused the end plate, Never again will I try to save a few bucks on a cheap replacement part. It's cheap for a reason. Perhaps tomorrow I'll bench test it, or perhaps drink heavily. Saving up for a new MC from reliable name brand. TRW folks?

There is much to be said for drinking heavily.