Like almost everything else, Agnetha’s brake pedal was solid when she arrived nearly 1 year ago. The master cylinder was heavily corroded and I thought it might not be saveable. There were a handful of different types of brake master cylinders used in Allegros over the years. Agnetha’s is, naturally, the rarest of them. It’s a tandem cylinder by Girling (so it controls two separate brake circuits: Built-in redundancy! Safety!) and replacements or reconditioned units are usually very expensive, but seal kits are available for reasonable prices so I wanted to see if I could rebuild it.
Earlier in the year I managed to remove the cylinder from the servo and disconnect the brake lines but couldn’t for the life of me remove the pistons (there are two: One for each circuit). The Haynes manual suggests shaking the body until the pistons come out, or blasting compressed air through the rear port. None of this worked of course, after 34 years of corrosion and coagulation of the hydraulic fluid, and on to the back-burner it went.
Recently I decided to have another go. First of all I filled the cylinder with WD40 and left it for several weeks, periodically topping it up. The piston would still not shift however. I ended up clamping the end of the piston in a vice and then, using part of an old jackstand, hammering the body off the piston with considerable violence for about an hour. Eventually it started to move but it needing beating to death the whole way.
That left the remaining piston. I was able to remove it explosively by sealing up all the other ports and shooting 8 bar of compressed air into the rear port. That made a hell of a bang and for a moment I couldn’t find the piston anywhere on the garage – it had landed in a shopping bag on the other side of the room!
Having got all the guts out I was able to clean the rust and check the seals. Some seals were good, others were not, so best to assume they’re all bust. The bore of the cylinder looks fine though – I think the gunk saved it. With a rebuild it should be good for another 122 thousand km, hopefully!
Projekt Agnetha 