Brake Vacuum Reservoir - yes or no?

[Snowy]

Hi folks! I'm undertaking a very major restructuring of my race truck in the UK, involving total removal of the structure behind the cab and going from indi rear suspension to a long travel four linked live rear end - and all by mid april! While doing so much work I thought I'd deal with some other stuff I'm not happy with, such as the brakes. Currently I'm running a passenger car pedal box with a vacuum servo (not sure what you guys call that over there!) and single master cylinder from a mid size van that has a bore of 23.2mm (13/16"?). This is acting on Mitsibushi calipers that are large diameter twin pots all round. While the 430 bhp, 5000lb (race weight including crew) truck will stop, the wheels won't really lock up.
I'd like to run (partly because it's amongst the stuff I'm gonna be retailing...) a CNC dual 1" master cylinder/pedal assembly with integral brake bias, with one master cylinder running front brakes and the other dealing with the rears. I'd like to do away with the vacuum servo, especially as losing one on a previous buggy lost 90% of my brakes on one event, and several of the admittedly smaller racers over here run like this - not saying that means they're right though! Given that the rebuild will lose some vehicle weight, and also that I'm going from 32" to 35" tyres (tires!), I'd be interested to know what some of the larger vehicles are running out there - at least of comparable size upwards.

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[Ol' Curmudgeon]

Lose the vacuum, consider smaller diameter master cylinders. Good Luck in '08!

[SimonSays]

Lose the vacuum, consider smaller diameter master cylinders. Good Luck in '08!

come on like hes gonna listen to someone name no brakes drake, lol

i went to the extra effort to hook up vacum brakes on my prerunner, took lots of work but man they work great, i like the way they feel better then our class 7 race truck that has all cnc pedals.

our class 7 truck weights 45000lbs uses 4piston calipers all the way around with 7/8 cnc master cylinders an can lock up all 4 35s if needed, i just personly think the brakes feel really spongy and like power better but in a case like yours it probably makes since to go with the cnc pedals could allow for better braking in your case.


ps. Does the pedal get hard and the brakes just not stop enough or is does it seem like there is just not enough pedal movement?

[Snowy]

NoBrakes Drake...thanks for your answer...the name irony is great! LOL! One thing I always look for is for an opinion to be backed up - what are your reasons?

SimonSays...I take it your class 7 weighs 4500lbs - not 45000! LOL again....man, that's some truck! Re your ps, the pedal doesn't get hard - that is to say solid - it just sort of stops moving, if that makes sense! We were running out of movement but modified this to give full travel in the master cylinder, and aslo went from a 7/8" cylinder to the current 13/16", which was new- this helped alot but not enough. I still feel there isn't enough volume in the M/C, especially as it is designed to run front calipers and rear slave cylinders in drum brakes when on a van. Part of me wants to run vacuum brakes, the other part doesn't - damned if I know! I definitely want the flexability of the pedal bias - I've been running a 0 - 57% bias valve in the rear line but didn't like it much.

[SimonSays]

NoBrakes Drake...thanks for your answer...the name irony is great! LOL! One thing I always look for is for an opinion to be backed up - what are your reasons?

SimonSays...I take it your class 7 weighs 4500lbs - not 45000! LOL again....man, that's some truck! Re your ps, the pedal doesn't get hard - that is to say solid - it just sort of stops moving, if that makes sense! We were running out of movement but modified this to give full travel in the master cylinder, and aslo went from a 7/8" cylinder to the current 13/16", which was new- this helped alot but not enough. I still feel there isn't enough volume in the M/C, especially as it is designed to run front calipers and rear slave cylinders in drum brakes when on a van. Part of me wants to run vacuum brakes, the other part doesn't - damned if I know! I definitely want the flexability of the pedal bias - I've been running a 0 - 57% bias valve in the rear line but didn't like it much.

i guess it would help if i could type. lol
yeah4500 lbs.
personaly we have not had much success with the bias adjustments on the pedals. never could get it to work. and when it did work it didnt realy do anything. always gave front and back the same braking even with it biased all the way to the front

[Ramsey_ElWardani]

All that the "vacuum servo" does is assist you in applying pressure to the Master Cylinder so that you don’t have to push as hard with your foot. The main reason that they are not used in racing applications is that race motors with radical cams do not create sufficient vacuum. One Class 8 that I have race experience with uses a stock Ford F150 Master Cylinder and Vacuum Booster with CNC 6 Piston Calipers and Coleman 12.2” Rotors. It has awesome brakes, but a relatively mild motor. Another uses the same hanging CNC pedal assembly with a GM Hydro-Boost attached and CNC 6 Piston Calipers and Coleman 12.2” Rotors. This also works great because this motor is too radical to develop sufficient vacuum. Another Class 8 I race uses the hanging CNC pedal assembly, CNC 6 Piston Calipers and Coleman 14” Rotors. It has awesome brakes as well but requires more pedal pressure to be applied.

The first thing you need to check is the brake fluid pressure you are getting at the caliper when the brakes are applied. It needs to be north of 1000 PSI with 1200 being optimal. If you are getting that much pressure, you need to look at things that effect brake torque – rotor diameter and material, pad size and material. If not, look at your Master Cylinder setup. It is more complicated than this, but this would be a good starting point.

[FABRICATOR]

The Hydroboost (Hydraboost per GM) is a little different. It can push harder than your foot. It is not rare to find M/C bore sizes in the 1.25" to 1.5" range in front of them. But that of course, would depend on your caliper bore size.

[Snowy]

Thanks guys - some food for thought there! Ramsey_ElWardani...thanks for that, I hadn't thought about taking a pressure reading on the brake circuits - obvious now you mention it! Only problem is, I just totally stripped and cut the truck clean in half, binning the back end, to build in the live rear end! Guess it will have to wait a couple months! :-)