Need help with updating brake master cylinder - The 1947 - Present Chevrolet & GMC Truck Message Board Network

NJM73 · 01-26-2019, 07:32 PM

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** · 01-27-2019, 02:52 PM

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 · 01-27-2019, 09:26 PM

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 · 01-28-2019, 01:54 AM

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.

01-26-2019, 07:32 PM	#1
NJM73 Registered User Join Date: Dec 2011 Location: Beavercreek, Oregon Posts: 34	Need help with updating brake master cylinder 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 __________________ '71 chevy suburban -all purpose rig- in progress '93 cherokee -dd- mild wheeler '65 chevy c30 c&c 12' flatbed -farm truck - future tow rig

01-27-2019, 02:52 PM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Need help with updating brake master cylinder 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? __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

01-27-2019, 09:26 PM	#3
69heavychevy Registered User Join Date: Jan 2007 Location: Garland,TX Posts: 431	Re: Need help with updating brake master cylinder 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. __________________ 80 C10 SWB Step Side - 350 w/ vortec heads, TH350, 12-bolt 3.73

01-28-2019, 01:54 AM	#4
Big Moose Registered User Join Date: Dec 2018 Location: Alberta Posts: 71	Re: Need help with updating brake master cylinder 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.



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