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705jmguy
April 24th, 2014, 04:30 AM
Hi guys

need some help on my power brake dual master cylinder conversion for 62 GMC drum front and rear

have used the Captains bracket, S10 booster and 68 chevy truck dual master

new master cyl. bench bleed, all lines are vacuumed bled

can not get much brake in the front at all, rears lock up easily

have read many different opinions on where to connect the front and rear brake lines

some say, front port ( furthest from firewall )of master should be for front brakes and closest for firewall should be rear, others say just the opposite

am very confused, does anyone have this set up using dual master and they work well?

what length of rod is needed for master cly to booster? and any chance my make up brake peddle rod ( peddle to booster) is not right length?

all help very much appreciated thanks JM

Funky61
April 24th, 2014, 05:47 AM
Did you adjust the front drums already so that they drag a bit when you rotate the drum?


Here is a thread from another forum with some info that may help.

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BarryGMC
April 24th, 2014, 06:54 AM
OK. It really does not matter how you hook it up. A dual pot master cylinder is just two separate hydraulic circuits. Internally connected. It's the same on a disk- drum setup. Only difference is that the calipers have larger pistons and cylinders and require a bigger resivore of fluid to compensate for the larger volume as the pads wear. So now let's visualize what's going on with the booster and dual pot master. With the engine running and no braking,engine vacuum is applied equally to both sides of the booster diaphragm. When you apply the brakes the firewall side of the booster is opened to the atmosphere. The rods from the pedal to the mc are continuos so vacuum + atmosphere forces the diaphragm towards the mc. This decreases pedal effort but does not increase output pressure of the mc. This may seem wrong but it's not. The input energy to the mc is the same. Boosted or not. Un boosted it may take 50 lbs of pedal energy to stop the truck. Boosted May be 25 lbs of pedal energy. The other 25 is from the energy transferred from the diaphragm to the rod. This energy is just atmospheric pressure acting against a vacuum. So if all of the hydraulic circuits are free of air the input energy should transfer to the wheel cylinders equally. This leaves only two options for your brakes to not operate properly. First you still have air in the front hydraulic circuit. Or the residual check valve is missing on the front curcuit. My bet is the later. I have seen this a lot on dual drum mc's. Barry

705jmguy
April 25th, 2014, 04:40 PM
Hi guys, thanks very much for your info

will work the next few days and try all your ideas JM :)

705jmguy
April 26th, 2014, 01:32 PM
hi Barry

how do I tell if my residual check valve is missing in my front part of my MC ?

very much appreciate your help here thanks JM

Vernski
April 27th, 2014, 09:13 AM
ON a dual MC if you run disc on front and drum on the rear you need a proportioning valve some are adjustable and some are not depends on the type of brakes and MC...Vernski:thumbsup:

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