power brake master cyl bore and booster diameter - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**MikeB** · 05-10-2019, 11:05 AM

I'm looking to convert my 82 C10 short bed to power brakes, and would like to use GM or exact replacement parts.

The truck's factory setup was JB1 manual brakes with 1.0" thick rotors on the front and 11" x 2" drums on the back. A while back I installed dropped spindles and the thicker 1.25" rotors, so I essentially have JB1 brakes with better heat dissipation (and larger inner wheel bearings) on the front.

Looking at the factory brake specification chart, what I want to do is upgrade to JB3 specs. The JB3 power master cylinder has a bore of 28.6 mm (1.125"). My JB1 manual master cylinder has a 25.4 mm (1.0") bore. So that's pretty straightforward.

How about booster? The GM spec table shows "240S". But what is its diameter? And is it dual or single diaphragm?

I assume I'll need a different pedal pushrod, but are there any changes to the pedal ratio? In other words, is all the under-dash linkage the same for manual and power?

Thanks.

**MikeB** · 05-11-2019, 12:55 PM

Found this at O'Reilly Auto Parts. More expensive than RockAuto, but I can easily return it and it has a lifetime warranty.

BrakeBest 50-1140

05-10-2019, 11:05 AM	#1
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,444	power brake master cyl bore and booster diameter I'm looking to convert my 82 C10 short bed to power brakes, and would like to use GM or exact replacement parts. The truck's factory setup was JB1 manual brakes with 1.0" thick rotors on the front and 11" x 2" drums on the back. A while back I installed dropped spindles and the thicker 1.25" rotors, so I essentially have JB1 brakes with better heat dissipation (and larger inner wheel bearings) on the front. Looking at the factory brake specification chart, what I want to do is upgrade to JB3 specs. The JB3 power master cylinder has a bore of 28.6 mm (1.125"). My JB1 manual master cylinder has a 25.4 mm (1.0") bore. So that's pretty straightforward. How about booster? The GM spec table shows "240S". But what is its diameter? And is it dual or single diaphragm? I assume I'll need a different pedal pushrod, but are there any changes to the pedal ratio? In other words, is all the under-dash linkage the same for manual and power? Thanks. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

05-11-2019, 12:55 PM	#2
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,444	Re: power brake master cyl bore and booster diameter Found this at O'Reilly Auto Parts. More expensive than RockAuto, but I can easily return it and it has a lifetime warranty. BrakeBest 50-1140 __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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