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55.2 3800 power brake booster vacuum #1332085 Thu Oct 31 2019 06:27 PM
Joined: Dec 2017
Posts: 31
E
ethanm90 Offline OP
Wrench Fetcher
My master cylinder wasn't building pressure so I thought this might be a good time to upgrade to power brakes. I just put this power brake booster I bought off ebay on my 1955.2 3800.

booster

It's mounted but still needs hooked up. I had to cut the pedal off the assembly to get it through the firewall but installing it so far has gone pretty smooth really.

I have a few questions I thought I might get input on.

1.) I still need to get vacuum from somewhere for the booster part. I'm sure I can pull it from anywhere but does anyone have a suggestion on where might be the best place to pull my vacuum from? Will it hurt anything if I pull it off the carb?

2.) This new booster moved the reservoir back so I can't use the fill hole in the floor anymore. I figure I need to cut a new hole so I can fill the brake fluid as needed. I hate cutting holes all over but o well. Any suggestions on the best way to cover a new access hole in the floor for the new one? I thought about the gasket off one of those cheap trailer grote lights from napa but I'm open for suggestions.

3.) The kit came with 2 residual valves. Do I need to use them? If so should I put them on the front wheels, back wheels, or order a 3rd one and put them on the two front and one on the rear before it tees off?


55 3800
Re: 55.2 3800 power brake booster vacuum [Re: ethanm90] #1332106 Thu Oct 31 2019 10:30 PM
Joined: Dec 2006
Posts: 134
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Elderly Iron Offline
Shop Shark
I just went around with CPP on that pedal issue. You CAN pull that one off and used your original pedal. You have to take eliminate the gap in the side play and make a small extension to reach the hyme joint for the booster rod. I'll have a video up soon.

It's a 4 disc set-up, so you need the residual valves to keep the shoes out near the drums. Just place them in line right out of the master cylinder. Drum brakes retain 8lbs pressure to overcome the return springs. Discs have no residual because it relies on rotor runout to move the pads back. You'll also want to eliminate the proportioning valve.

Re: 55.2 3800 power brake booster vacuum [Re: ethanm90] #1332142 Fri Nov 01 2019 11:12 AM
Joined: Dec 2017
Posts: 31
E
ethanm90 Offline OP
Wrench Fetcher
Thank you for explaining this to me. I love being the dumbest person in the room because I can learn so much. I have drums all the way around, so it sounds like I need them. I hooked it up using the combination valve provided. Is it a problem if it's left on so it's less plumbing? That's probably why I only have 2 residual valves, one for the front and one for the back. I could probably leave it as is and add another residual valve to the back couldn't I?


55 3800
Re: 55.2 3800 power brake booster vacuum [Re: ethanm90] #1332158 Fri Nov 01 2019 02:11 PM
Joined: Dec 2006
Posts: 134
E
Elderly Iron Offline
Shop Shark
The proportioning valve reduces pressure to the rear cylinders. On passenger cars and especially light trucks, the majority of the weight it transferred to the front axle during braking. Your truck is already designed to have equal hydraulic pressure at front and rear, and most of the braking is done on the rear because it's supposed to have a load on it. If you run the proportioning valve, you will practically eliminate rear braking.

Re: 55.2 3800 power brake booster vacuum [Re: ethanm90] #1332269 Sat Nov 02 2019 12:51 PM
Joined: Jun 2011
Posts: 2,973
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EdPruss Offline
Shop Shark
GM 3/4 ton pickups and others have a proportioning valve in the rear hooked to the axle such that when loaded, more braking power is sent to the rear, east to adapt to other vehicles.

Ed


'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.

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