Vac-Boost 4WDB Users, come on in.

[BigCountryx]

Those of you that didn't jump on the Hydroboost bandwagon, how are your brakes after the rear disc swap. I have heard some can't stop and others say they can lock up their 40's. If their is a Vac-Boost combo that can lock up your tires, I don't see the advantage of going hydro. Your braking can't be better, just the brake pedal must be easier to push. Does this thinking make sense? Let me know what ya'll have found.

What I'm thinking is my truck has the JB5 (1/2 ton vac system) and I am not satisfied with the performance. I can stop my truck well, but with a long pedal stroke and at no time can I "lock" my tires. I feel like I am on the verge, but It won't lock. I feel like my problem is that I need more volume for the large D60 calipers and new rear calipers. What I am thinking is that if I upgrade to the JB7 Booster and Master cylinder (3/4 ton vac boost system, this system shares the same master cylinder as the 3/4 ton hydroboost system) , I should get the extra volume that I am looking for. Has anyone else done this or am I gonna be the guinne pig? I'm sure someone has tried it.

 

[randyb12]

I think it is allways a good idea to get the master cyl and booster from whatever the axles came out of. For example I got my axles out of a 1 ton truck and put them in a 1/2 ton. Of course the brakes are different so I grabbed the booster/master cyl at the same time.
Even with the 46" Baja Claws braking is just like a stock 1/2 ton truck.

 

[seschev3]

If you are already going to swap parts in, just get the hydra-boost out of a 1-ton and call it good. I have done the swap in my past 3 rigs and it always has made a huge difference. The whole setup usually runs me between 75 and 125 from the yard. Get the master, boost, pedal, steering pump and brackets, and hoses. I even have spare sets because they are so great. Plus if you go radical cam or anything that affects vacuum at or off idle you may regret staying with the vacuum boost. Just my experience and opinions.

 

[Blue85]

I did the swap with the stock master cylinder. I could just barely lock up the tires, but the pedal travel is really far. Then I swapped in the Caddy 4WD mater cylinder and everything was exactly the same. Part of my problem is that my rear calipers are the same as the fronts. This means they take tons of fluid to engage. The smaller the piston in your calipers, the higher your pedal will be.

The reason hydroboost works so well is that it gives you enough assist to use a larger bore master cylinder. The basic premise of hydraulics is the the force multiplication through the system is equal to the ratio of the surface area of the two pistons (MC and caliper). As you make the MC piston larger, you move more fluid, but loose force applied. By using the hydrobooster, you can apply more force to the MC and compensate. I'm just not sure how well you can stop with the engine stalled.

So if you want to avoid going hydroboost, use the smallest calipers you can out back (i.e. Eldorado), make sure the pedal and linkage are adjusted to take out the slack and get your front/rear proportioning right. The other option I have considered trying is putting a residual pressure valve on the rear line (a few psi) to get less fluid flowback and get the brakes to begin engaging with less pedal travel. I suppose the same could be done to the front. The key is to do it enough to reduce fluid transfer, but not enough to put drag on the brakes.

 

[BigCountryx]

Isn't that basically what the stock disc/drum p-valve does? keep some residual pressure on the rear brakes. And I allready put 1/2 ton callipers on the rear to reduce some of the volume needed. I think I'm gonna try the JB7 setup and see how it does.

 

[Blue85]

Now that I think about it, the hydroboost can either use a larger bore MC or the pedal can have less mechanical advantage on the rod. The effect is the same either way: less pedal travel and the pedal is harder to push. The stronger booster compensates for this.

 

[BigCountryx]

As an Update, I have Power bled my brakes again. My setup is D60 Callipers up front and my old 10b callipers out back on my 14ff disc swap. My pedal still has more stroke than before, but very good feel. I replaced my MC w/ a new 1/2 ton unit (direct replacement, didn't upgrade due to lack of availability), bench bled it and pulled my ABS Module. Everything else is stock including the proportioning valve. I had stomped the brakes hard at slow speeds (~25mph) and only came to a quick nose diving hault. Felt good, but thought baybe it should be better because the tires didn't lock. Well last night I was running about 45mph and no one was around so I figured it would be a good time to check out braking. All I can say is wow, the howl of 4 big swampers locked up is cool I now feel that my brakes will stop me as fast as any other setup I could go with, locked up is locked up, right? I can't see where more clamping power from hydroboost would help, but maybe that's just me. The extra .5" - 1" stroke doesn't effect performance at all.

 

[mouse]

When I first did the rear disk swap I used the 3/4 ton master cylinder and vac booster. It worked similar to how you described, long pedal throw, and on the verge of locking. If I really put my foot into it, I think I could have locked them (I think). The MC I used was for the 11" brake setup on the 3/4 ton 14BFF.

I won't tell you what I run now...or how much better it is...

 

[readymix]

Here is my setup. And it will fully lock all 4 35's anytime i want. Only done it a few times as it shakes the hell out of the truck

>>>D-60, Stock calipers, OEM rotors, $9.99 pads, new flex lines
>>>14FF, 1975 front 3/4ton 4x4 calipers, SKY brackets, OEM rotors, $16.99 pads, custom SS line, Wilwood Prop Valve
>>>Stock 1/2 ton MC, Unmodified original prop valve.

Like i said it will lock the wheels as is, The Wilwwod prop valve out back is set to lower the pressure quite a bit, as the rears lock pretty easy without much weight back there.