The old master cylinder was leaking badly at the pedal end under the rubber cover. It had been rebuilt on a number of occasions and with new ones available from Cavalitto I thought it would be safer to get that as opposed to trying to get it repaired again.
Aprilias had two different brake systems though the years. Originally they had Lockheed wheel cylinders and a Marelli master cylinder, all with 1″ imperial internals and, of course to add to the confusion, imperial nuts and bolts. Later in 1946 2nd series models from construction number 12778 they changed all parts to Italian SABIFs.
The new master cylinder from Cavalitto came complete except for the two brake line fittings, it cost 250 euros plus shipping
The top banjo fitting (TAV 28 38-46112) that has the line from the brake fluid reservoir was a straight swap, but the end pipe fitting nut needed to be adjusted, it only screwed into the end of the new part one thread.
To remove the nut meant cutting the flared end off the brake line. Which then meant I had to buy a copper pipe flare tool and learn how to use it! The purchase wasn’t wasted as I found the flare on the top line also needed to be re-done.
The master cylinder is fairly simple to install, just three bolts screw onto a bracket in the bottom of the firewall, attach the two brake lines and connect the pump control pin to the brake pedal. Then bleed the entire system, make sure you use the same DOT fluid, I use Castrol DOT 4.
Don’t make the same mistake I made and not check the clearance on the brake pedal . When I did a road test the brakes kept locking on. After much fruitless head scratching a conversation with the knowledgable straightened me out. The pedal wasn’t returning far enough which meant the piston in the master cylinder wasn’t clearing the internal fluid return hole and releasing the pressure. A couple of turns of 38-43518 (TAV 27) and they worked fine.