1970 RM7500 clutch slave cylinder question

[WE7X]

I have seem listings for a clutch slave cylinder for a '65-66 3500 series that looks like the one on my 1970 RM7500 (rear engine) chassis ( 401M motor) and Clarke 5/2 transmission. Rock Auto for one...in Wagner and Dorman brands.
I am wondering if anyone has a way to determine if there might be some minor differences in one for a 1970. Mine sat for several years, and the piston is now seized at the bottom of the bore. The bore is so rusty that I do not think it is repairable, and I might not even be able to salvage the piston. I have a second slave cylinder, but it is in the same sorry condition.
I can have it bored and sleeved if necessary, but if the '65-66 is the came cylinder, I want to go that way
I see two listings, which show slightly different bore sizes ( 1.063 and ?0.96? or something similar) I have not tried to measure the bore of the original quite yet., but I see a '15/16' cast into the Delco housing, so i know it is a nominal 1" bore....give or take a lot of rust.
Any input is helpful.
or call (425) 466-0884
Rod Johnson

[LEWISMATKIN]

Use the ".96" listed cylinder--15/16" is .937" so the ".96" cylinder will be closest to the original. If you go with the larger bore--"1.063"=1 1/16" can cause a fluid displacement problem especially if the bore of the master cyl. is smaller than the slave cylinder. I hope this helps you in your decision.

[WE7X]

Lewis,
Thank you for that input, and I agree with the diameter vs. fluid transfer issue.
I found a Wagner SC45289 listed at Rock Auto; as fitting a '64 or '65 3000 series, and the picture looks correct. The alternate item ( Dorman CS36124 ) has the slightly larger bore, and the input fitting location is on the top, vs. on the bottom in my particular setup.
I then found a source one at Centric, and a bit less expensive than Rock Auto.
It has been ordered.
I would have been fine with having the local shop putting a sleeve my old cylinder (which they would do for the same cost), but we had not yet found a replacement piston. I am sure both of my original pistons will be in pretty bad shape if I can even get one of them out in one piece.
Now off to the local NAPA for some new supply ( remote reservoir setup in this rear engine chassis) ) and return hoses, and to get a new pressure hose fabricated.
It seems to very rare for any place to have any parts listings or information on anything bigger than a 4000 series chassis in a roughly 50 year old GMC. Rod J
Issaquah, WA

[WE7X]

This response was from a very old posting, but the new slave cylinder worked fine after finding a small hydraulic leak.
I got an offer I could not refuse, to sell the old monster, so it was necessary to finally fix the clutch issue.
Turns out the hydraulic line had worn a pin hole at a frame clamp, so I could not build any significant pressure to operate the slave cylinder. The small amount of hydraulic fluid leaking ran along the frame rail for several feet, to a spring hanger, where it eventually dripped off a spring shackle, into gravel. It was very hard to even see the wet spot, and it took a lot of time to finally find the source of the leak.
The old motor home conversion (1970 RM7500-a school bus chassis with a 1954 Spartan Travel trailer for a body) is 38 feet long, so the clutch hydraulic line was over 30 feet of tubing ( rear engine configuration ) .
A roll of Ni-Cop tubing and a couple of hours work threading it through the frame cross members solved the problem.
The old motor home drove away just fine, with it's 401M purring nicely.
I have a spare Allison automatic transmission and adapter plate, starters and a couple of magnum heads, and a magnum exhaust manifold remaining, if anybody is looking for something like that in the Seattle area.
Rod J
Issaquah, WA