I would be inclined to clean it up ,pull it apart and inspect the bore as previously said , they are not hard to rebuild , it may only require a hone and a new kit .

Brake Smart in Fairy Meadow are quite helpful if you decide to get it honed or sleeved.

55 MONTAGUE STREET, Fairy Meadow, NSW, 2519


Sent from my iPhone using Tapatalk

_________________

68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

I agree with peterb. drum brake M/C had a valve to hold pressure against the wheel cylinder cups, for better application. Disc brake M/C didn't have the valve because they didn't have return springs like drums and had to self release.

_________________
Dalmeny

I thought all brake master cylinders had a non return valve.
At least all my cars do and the rubber kits always include the valve

_________________
1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

It is not a non return valve, it is a return flow restrictor. Disc brakes will work with or without it.

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks all for the help. I've ordered a "drum" tin type GMC171 complete replacement unit. I'll probably keep the old one as a spare if it can be rebuilt.

_________________