Brake Issues:::: HELP..

[Erik Carlson]

Hey guys~ I searched the forum and can't find this answer. I know I'm missing something here. I've just installed the Duff Power Boost Brake system and the brakes won't bleed. I can't even build up any pressure to get air bubbles OR constant stream outta the bleeders. 1) Is there an adjustment in the booster you have to do when you install? (all the linkages look like they're perfect length going into the booster) 2) Do you have to start the engine for a while to build up the vacuum enough before you get pressure to the individual bleeders at each wheel? (Just rebuilt the Bronco and haven't even started the new engine yet. I wanted to get the brakes set before the engine breakin). Thanks Allot.

 

[BG's 68 Bronco]

I don't know if this will help, but did you bench bleed the MC if it is new? You shouldn't need to start the engine.

 

[Erik Carlson]

BG- yes, bench bled. I'll try again tonight and take some pics if I can't get er done this time. thanks.

 

[BG's 68 Bronco]

did you have disk brakes upfront originally? Also do have someone helping you bleed the brakes or a one man bleeder kit?

 

[Erik Carlson]

just 4 drums (disks are my next project). Bench bled while connected to the booster with buddy inside pumping.

 

[BG's 68 Bronco]

Are you using the original MC or one that came with the kit?

I'm guessing a bit here but, a disk/drum MC will not have a residual pressure valve on the disk side where as the drum/drum would have it in both. this is because the piston keeps force on the brake shoes, so they don't move away from the drum, otherwise yuo would have to pump the brake to build up the pressure.

I'm just guessing here, as I have never encoutered this myself this is just what i've read on this site and may be a possible cause to your lack of bleeding?!?!?

 

[stlkikn]

Have you adjusted all four drums yet??
Drum brakes will not bleed properly unless they are adjusted.

 

[72Sport]

Are you using the original MC or one that came with the kit?

I'm guessing a bit here but, a disk/drum MC will not have a residual pressure valve on the disk side where as the drum/drum would have it in both. this is because the piston keeps force on the brake shoes, so they don't move away from the drum, otherwise yuo would have to pump the brake to build up the pressure.

I'm just guessing here, as I have never encoutered this myself this is just what i've read on this site and may be a possible cause to your lack of bleeding?!?!?

Disc master cylinders have residual pressure valves also. They just have less because of the large bore of the calipers. Pressure on the rotors is just pressure times area or about 15# on each piston in the caliper

Lets do a little math to address the shoes against the drum theory. I have also read that residual pressure in a drum system is 10 psi. The area of each front piston is .9940 square inches. Ten pounds per square inch times .9940 square inches equals 9.94 pounds of force. The last time I stretched a drum return spring I think I must have exerted about 40# of force per spring to hook them up. There is no way possible that 10 pounds of residual pressure can overcome that 40 pounds of spring force to move the shoes against the drum.

I have read that the residual pressure is there to keep the cups in the cylinders sealed against the inside of the wheel cylinder to keep out moisture since brake fluid is hydroscopic. There has also been conversation saying that the residual pressure valves are there to keep fluid from running back into the master cylinder in older vehicles before the advent of "hanging pedals". The Bronco has hanging pedals. I think an added benefit of residual pressure valves is that it maintains a small amount of pressure and brake fluid in the system so you don't need a bigger bore master cylinder. When you remove the residual pressure valves you need more stroke in your pedal system. We solve the long stroke problem by increasing the master cylinder bore. We solve the higher pedal force by trying to add a vacuum booster. When we don't like that solution we add hydroboost and that fixes everything until your engine dies and you have to make it stop with that big bore master cylinder without help other than your size 12's.

Talk about on and on and on. Just my 100 cents;D I think its called a filbuster and a lot of run on sentences and paragraph. Sorry for messing with your thread.

 

[BG's 68 Bronco]

Good info 72sport, Like I said I'm just guessing, but using what I've read.

Stilkikn, That is probably his answer!

 

[Apogee]

Not to contradict you 72sport, but I've always read that residual valves were for Master Cylinders lower than the calipers (2#) or drum brake systems to offset some of the shoe spring force. Keep in mind that the wheel cylinder has some mechanical advantage over the springs due to the greater leverage ratio distance to the pivot...not by a factor of 4X, but it is greater.

http://www.wilwood.com/Centers/Information/question_answer/10.asp

I'm thinking the shoes need to be adjusted. Let us know what it was when you get it figured out.

 

[Erik Carlson]

Thank you for all contributors... I ended up finding that my front left bleeder was stripped. I pulled front rotors to do a complete brake job and found the front right to be way grooved. So, I guess it was good this happened. Should be back to bleeding tomorrow night.

 

[POWERBRAKEBOB]

Check Valves

The Factories Never, Ever Used A 2lb Check Valve. There Was No Need For It. 10lbs Were Used In Single Master Drums Systems To Keep The Seals Tight Especially With Out Of Round Drums. The Factories Never Had A Master Mounting Lower Than The Calipers. The Hot Rod Disc Guys Used Check Valves To Preload The Calipers, Because They Were Trying To Get Away With Using A Tiny Master, And No Booster. The Tiny Master Doesn't Deliver The Volume Of Fluid To Fill 2 3" Calipers Pistons. The Factories Invented Dual Dia Boosters To Crank Up The Pressure When They Went From A 1" Drum Master To A 1.125" Disc Master. Once They Got The Pressure Up, It Overpowered The Drum Brakes, Which Lock Up At Around 600psi. So. The Prop Valve Limits The Rear Pressure To 600psi, Allowing The Front Pressure To Rise Over That To 1200psi. The Problem In This Case Is The Master/booster Is Not Completely Releasing, And Allowing The Fluid To Suck Down, Or It Could Be As Simple As Leaving The Cap Loose When Bleeding. If The Cap Is Snapped Down Tight, It Will Not Allow Th Efluid To Draw Down Into The System, As A Vacuum Is Created When You Pull The Fluid Down Too Fast In The Reservoir