There is a lesson buried in this episode and it is not a subtle one. The CB750F seized caliper piston that I had been fighting since removing the rear brake caliper weeks ago, the one that survived PB Blaster soaks, ultrasonic cleaner runs, breaker bars, Allen keys, Torx drivers, and increasingly desperate attempts at rotational force, got sorted at Jeff’s shop in roughly 35 minutes.
The right tool for the job is not always a tool you can buy. Sometimes it is just knowing someone who knows what they are doing.
Table of Contents
The Seat Pan Is Still a Problem
Before getting to the good news, a quick update on the seat situation from finding out how bad the CB750F seat pan really was. Two fasteners on the seat strap completely exploded when I tried the hand impact on them. The JIS head on both screws was so corroded that it just disintegrated on contact, leaving spinning studs with nuts fused to them. I cannot drive them, I cannot turn them, and I cannot grab them cleanly with vice grips.
The plan now is to use a bimetal blade in the oscillating tool to cut the nuts off and drive the screws back through the seat pan. That should at least free the strap so I can get the foam off and finally see the full extent of the damage underneath. The trim piece that was giving me grief last session turned out to be simpler than I thought once I found the elongated cutout on the back and drove the frozen pin through with a mallet. That piece is off and cleaned. The fasteners on the seat pan itself are the remaining obstacle.
The seat pan is bad. I can see it getting worse the more I expose. The JB Weld steel stick repair plan is almost certainly off the table at this point. Whether I try to fabricate replacement patches myself, source an OEM pan, or go with a reproduction seat pan and adapt the cowl and trim to fit it, I do not know yet. An OEM F model pan is proving very hard to find since the F seat is different from the K Model.
Dropping Off the Tank and Calipers at Jeff’s
I loaded up the gas tank and rear brake caliper and drove them out to Jeff. The tank he is keeping until he can fit the patch and weld work into his schedule, which could be a week or two. He is confident he can cut out the bad metal, weld in a patch, and pressure test it afterward. He also mentioned that the dent in the tank might be popable once he cuts into the bottom, which would be a nice bonus if it works out.
The calipers were a different story. Those came back with me the same day.
How Jeff Fixed the CB750F Seized Caliper Piston
I want to walk through exactly what Jeff did because the technique is worth documenting for anyone dealing with the same problem on a vintage bike.
He put the caliper in a vise, applied heat directly to the piston with a torch, and then once everything was good and hot he dumped cold water into the piston to shock it. The thermal shock is what breaks the corrosion’s grip. Once the piston moved even slightly he got a wrench on it, worked a screwdriver in as a lever to assist the rotation, and it came out. Start to finish on the first caliper, not long at all.
The second caliper took a little more work because of the geometry. The piston sits down inside, which left less room to maneuver. Same basic approach though, heat, water shock, rotation, and it came free.
He also cleared the bleeder valve, which had been plugged and corroded, cleaned up the threads on both the valve and the caliper body, and ran a cylinder honing tool through the bores to knock off the surface corrosion inside. Not aggressive material removal, just a few passes to clean things up. That saved me a significant amount of time with a wire brush trying to do the same thing by hand.
The Bad News on the Pistons
Both pistons are shot. The corrosion went through the chrome plating and into the base metal, which means they cannot be reused even with new seals. I need replacement pistons.
The rebuild kit for these calipers, which covers just the rubber parts, two seals, two dust covers, and a bleeder valve cap, runs about $62. If the rubber alone is that much I am not optimistic about what the pistons themselves are going to cost. I need to do some research and see whether a complete kit with pistons and seals is available and what the damage looks like. The front calipers still need to come off at some point too, which is a separate battle for a later session.
What Comes Next
The tank is in Jeff’s hands and should come back in a week or two. The calipers are back in the garage with bores cleaned out and pistons removed, waiting on replacement parts. The seat pan situation is still unresolved and is probably the most uncertain thread on the whole restoration right now. And there is still a full list of things to come off the front half of the bike before I can think seriously about reassembly.
But getting those pistons out and the tank dropped off in the same afternoon felt genuinely good. Two problems that had been sitting on the open thread list for a long time are now either resolved or actively in someone else’s capable hands.
Ride With Me
If you want to see how the tank repair turns out and what the replacement caliper pistons end up costing, subscribe to the WaltInPA YouTube channel
so you catch every update as it goes up. If you know someone who has been fighting a seized piston on an old bike and getting nowhere, send this one their way. The answer really might just be heat and someone who knows how to use it. And if you want to follow along with the full CB750F Restoration as it keeps moving, come hang out in the WaltInPA Discord where good people talk through exactly this kind of thing.
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