Brake Cylinder Modification

[Graham Dix]

My '79 bgt failed its MOT due to the rear brake cylinders sticking ..they seemed OK during the pre MOT checks but I think the force applied during the testing must have pushed the pistons out further than usual, which due to the tinniest amount of surface rust on the ends of the pistons (not enough mgb’ing due to my other hobby ..triathlon) resulted in them sticking during the test. I brought two replacement cylinders (GWC1122 Z) the same day from a local supplier but I couldn't get the brake pipe unions to fit (so much so that I was convinced I had been supplied the wrong ones). The supplier was at a loss (a UNF male fitting suggested there wasn't a problem), eventually suggesting I file the end of brake pipe’s UNF nut as it may have splayed (...umm?). Back home and still with no joy with the replacement cylinders the wheels were refitted and the bgt rolled back into the garage.

A closer examination of the parts confirmed that the OEM (faulty) ones had a chamfer around the hydraulic connection, but the replacements ones did not (see photo). I was confident that this was the issue as it's not the first set I've replaced on the bgt and I’ve never had a problem before, so I plugged the hole (blu tac has more uses than you think) in each of the new cylinders to avoid metal filings entering the cylinder’s bore and chamfered each using a 12mm drill bit ... not pretty I'd admit, but I don't have a piller drill just a drill stand ... but the chamfering did the job (see photo) and I’m back up and running. I'm not sure if this issue only applies to copy parts fitted after a certain year of bgt manufacture, but hopefully it will help someone? Obviously if you have time to shop around purchase OEM, or copy parts with a chamfer. If you can only get copy parts without a chamfer this modification will work for you.
Photos show: Copy part, Old OEM part and, Modified (chamfered) copy part.

[Peter Cresswell]

Hi Graham
As I am sure you know but to be clear for others, the 'Z' at the end of the part number indicates it is an aftermarket copy and not an OEM part. Hence the slight differences as you have found out. The copy parts are often the source of dissatisfaction over the quality of parts supplied these days. Most (if not all) of the main parts suppliers will have the OEM part in stock as well, in this case the part number is GWC 1122, which costs around £21.00 each compared to GWC 1122Z at around £8.50 each, so a tempting buy until you get an inconvenient failure!
Incidentally (and again to be clear) the roadster uses a different rear wheel cylinder to the GT due to the extra weight on the rear of the car. These are GWC 1103, and considerable cheaper than GT ones for both OEM and aftermarket. The rollpin spigot in the pictures stops the incorrect one being fitted to the car - unless the backplates have been changed during restoration! It also means you have to swap backplates if you fit a GT axle to a Roadster or vv, which are otherwise the same.