Need help with updating brake master cylinder

[NJM73]

I want to put a newer brake master on my 85 1 ton. Looking for one with a screw on lid plastic reservoir style. Reason being is that I am tired of going through gaskets and tension springs on my master. I keep the fluid level low too hut nothing seems to be a permanent fix.
Truck has hydroboost
Just tried early 2000s vac and hydro masters but both had bore too big to fit into the hydrobooster. The 90s chevy ones dont look like they would be much better than what I have now.
Wondering if there is another application that would have a master with a smaller case bore diameter. Trying to stay Chevrolet since everyone else went to metric fittings off the masters

[hatzie]

The GMT800 and newer master cylinder reservoirs make pressure bleeding the brakes a breeze. That three cam cap system would be very nice to have on the older trucks.

The GMT800 has Disc/Disc brakes vs your Disc/Drum brakes so the Master Cylinder bores will be biased differently from the squarebody and T400 master cylinders.

As you discovered, the tail of the T800 master cylinder is larger too... You may be able to just fit the Hydroboost and MC from a GMT800 with JD5 or JB5 or JD7 or whatever RPO HD brakes you have but that won't cure the differing bias...
If you swap in the AAM axle from the same GMT800 that'll get you close enough that it'll work... You get to re-cable the E-brake with the AAM swap along with moving the spring perches and shock mounts and any needed brake line changes.

Kindof a slippery slope.

Can you fit the plastic GMT800 reserviour onto an RV30 series Master Cylinder with the big square plastic reserviour that's setup for the same RPO brakes you have?

[69heavychevy]

I went through this same issue when swapping over to hydroboost. I wanted to use an aluminum MC with the plastic reservoir. Mostly for the ease of keeping an eye on brake fluid level and avoiding the rusty look of the original cast iron MC.

The piston bore on the various aluminum MC were all too large to mount into the HB unit. So I just ended up going with the cast iron MC unit it called for.

The GMT400 HB units mount up perfectly but the issue is the pedal rod is different. Those units do use an aluminum MC, they're essentially identical with minor external differences. I'm sure a newer HB unit can be made to work fine on these trucks, but I just didn't feel comfortable experimenting with my brakes.

[Big Moose]

Installed a HB out of a 2005 2500HD on our 85 1 ton but had to modify the rod that connects to the brake pedal, had to lengthen it. also tried to install the MC with the plastic reservoir as well, but it was too tall, not enough room for closing the hood. so had to put the old MC back on.