Hydroboost with disc/drum - wheel cylinder bore size
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Laughing Gas
Joined: Jan 2012
Posts: 882
Likes: 70
From: Santa Clarita
Hydroboost with disc/drum - wheel cylinder bore size
I have a 1975 crew cab 4x4 with 35" tires, 4" lift, a 78/79 disc brake front axle, and stock rear drums (12" x 2.5") using a hydroboost unit and master cylinder from a 1989 F-450 SuperDuty, with a Wilwood proportioning valve.
I've been running the stock 15/16" wheel cylinders but have the chance to switch to a different size due to needing to replace the rear shoes. The other day, I noticed a bit of brake fluid had dripped from my rear passenger wheel & I felt a drag and difference in my brake pedal feel when I moved my truck out of the driveway.
I had to back off the star wheel to remove the brake drum. This is what I found!
For those of you running front disc and rear drum with hydroboost, what size bore rear wheel cylinders are you using? Anyone switch rear wheel cylinder bore sizes and have feedback on the impact? I found this post from @ford390gashog that says to use GM style 1 - 3/16" but no real explanation as to why this is recommended. From 15/16" to 1 - 3/16" is a pretty big jump that I'm wary of making without a clear explanation of the benefits/tradeoffs.
I'm inclined to step up to a 1" rear wheel cylinder bore size, which is a 13-14% increase in bore size vs the 60% increase in rear wheel cylinder force a 1 - 3/16" would be. I don't tow often, but occasionally load up the bed or tow up to 7.5k lbs.
I've been running the stock 15/16" wheel cylinders but have the chance to switch to a different size due to needing to replace the rear shoes. The other day, I noticed a bit of brake fluid had dripped from my rear passenger wheel & I felt a drag and difference in my brake pedal feel when I moved my truck out of the driveway.
I had to back off the star wheel to remove the brake drum. This is what I found!
For those of you running front disc and rear drum with hydroboost, what size bore rear wheel cylinders are you using? Anyone switch rear wheel cylinder bore sizes and have feedback on the impact? I found this post from @ford390gashog that says to use GM style 1 - 3/16" but no real explanation as to why this is recommended. From 15/16" to 1 - 3/16" is a pretty big jump that I'm wary of making without a clear explanation of the benefits/tradeoffs.
I'm inclined to step up to a 1" rear wheel cylinder bore size, which is a 13-14% increase in bore size vs the 60% increase in rear wheel cylinder force a 1 - 3/16" would be. I don't tow often, but occasionally load up the bed or tow up to 7.5k lbs.
Cargo Master
Joined: Dec 2008
Posts: 3,209
Likes: 319
From: SW PA
Not sure if 1 3/16" would clear the backing plate...1" i think is D70/1ton size which I used with success with a 1 ton dual diaphram vacuum booster in a 3/4 ton truck on 37's.
While the above is not a great data point for your question, i suppose if you could get the 1 3/16" deal to fit, if it was too much rear brake causing lockup, you could tune it in with an adjustable pressure reducing valve.
While the above is not a great data point for your question, i suppose if you could get the 1 3/16" deal to fit, if it was too much rear brake causing lockup, you could tune it in with an adjustable pressure reducing valve.
Hotshot
Joined: Feb 2004
Posts: 10,726
Likes: 2,584
From: The Shenandoah Valley
Not your set up, but my '77 which I redid with F-350 sized1-1/16" bore MC / dual booster, and used my stock 11" dia. rear brakes, etc, I installed1-1/16" F-350 rwcs first, but in a hard stop they would lock pretty quickly. I cleaned & reused my stock 15/16" rwcs ever since, but not too long ago bought new 1" F-250 size ones for hitting the sweet spot. Bigger pistons use more travel to apply, but apply harder with same pressure.
Last edited by tbear853; Today at 11:53 AM.
Tuned
Joined: Feb 2020
Posts: 459
Likes: 234
From: South Central Mo
My 79 F350 4x4 has 1" wheel cylinders.
Has a Dana 70-1u rear end with 37" rubber & hydro boost brakes. I can just about launch someone through the windshield if I stomp on the brakes hard & theyre not wearing a seat belt.
Has a Dana 70-1u rear end with 37" rubber & hydro boost brakes. I can just about launch someone through the windshield if I stomp on the brakes hard & theyre not wearing a seat belt.
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