Excessive Pedal Travel

[dbranger94]

I have a '94 Ranger race truck that I am building and have converted it to 4 wheel disc brakes. I installed 12" discs and dual piston calipers on the front and '95 Mustang 10.5" discs on the rear. (In place of 9" rear drums and single piston 10.5" front discs.) My problem is that my brake pedal travel seems excessive now. It is about 1/3 to 1/2 travel before the brakes "engage", but as soon as they do it stops very quickly. I have read that residual valves can be used to cure this on vehicles where the brake MC is low mounted in the chassis and brake bleedback is a concern. My question is, would 2lb residual valves front and rear help with my problem even though the MC is in the stock position? Also, I believe that my MC is large enough to handle the setup as it has a 1" bore. Any suggestions would be appreciated.

[MonzaRacer]

I have a '94 Ranger race truck that I am building and have converted it to 4 wheel disc brakes. I installed 12" discs and dual piston calipers on the front and '95 Mustang 10.5" discs on the rear. (In place of 9" rear drums and single piston 10.5" front discs.) My problem is that my brake pedal travel seems excessive now. It is about 1/3 to 1/2 travel before the brakes "engage", but as soon as they do it stops very quickly. I have read that residual valves can be used to cure this on vehicles where the brake MC is low mounted in the chassis and brake bleedback is a concern. My question is, would 2lb residual valves front and rear help with my problem even though the MC is in the stock position? Also, I believe that my MC is large enough to handle the setup as it has a 1" bore. Any suggestions would be appreciated.

Well is there much difference in Mustang to ranger master cylinder? Some of the Fords ran stepped MC. also do you have an adj prop valve in rear line?
and yes 2 lb checks are used to keep pedal travel to a minimum on disc setups, 10 lbs are used on rear drums.
Also the Ranger master if NOT a stepped bore may be too large and you may need to drop bore size as in from 1 in to 7/8in, what this does is allow the pedal stroke to raise pressure faster.
See disc brakes operate on pressure, not volume, drum breaks run less pressure but depend on volume over pressure as they have to move more.
and by moving figure it this way, disc only have to move a few thousandths, drums may have up to .100" or nearly 1/8th in, but then this also means drum brakes need adjusted but then you get idea.

[Apogee]

First off, rotor diameter has nothing to do with pedal travel, it is strictly related to the hydraulics of the system. Since this is a race truck, we can't really assume anything is stock, so what's your pedal ratio? Is this a manual installation or power? If power, are you running vacuum assist or hydroboost? If hydroboost, are you running the GM/Bosch booster, Ford Mustang type booster, or something esle? Also, you've told us your front calipers are 2-piston units...which doesn't tell us anything. What application are the front calipers from, or even better, what size pistons do they have since we don't really need to know what they came off of in order to determine a suitable master cylinder.

The rear '95 Mustang GT calipers have 66mm [2.6"] diameter pistons with an approximate piston area equal to 5.3 square inches. Provide the other details noted above and we can provide more than just a guess as to what you system needs to fix the pedal travel issue. Generally speaking, a larger bore MC will shorten the stroke.

Tobin
KORE3

[dbranger94]

I'll do my best to answer all the questions.

The master cylinder is stock '94 Ranger 1" straight bore (No Step). I believe it does have a spring inside to separate the front and rear sections to provide bias. There is no prop valve in the rear line other than the RABS valve. Here is a link to the MC.

Pedal ratio is 4:1. It has 12" overall length with rod attached 3" down from pivot.

The truck is using the stock 8" vacuum brake booster.

The front calipers are '95-'97 style Ranger calipers and have twin 46mm (1.811") pistons. So, about 5.15 square inches per side.
Link

Rear calipers should be single 38mm (1.5") pistons. So, about 1.77 square inches per side.

What I find interesting is that my setup is basically identical to an SN95 Mustang, with the exception that the Cobra only had 1.5" front caliper pistons and a 15/16 bore MC.

[dbranger94]

Ok, I did some rough calculations.

Line Pressure
150lb input force on pedal = 600lb at the brake rod, with 4:1 pedal ratio.
My 8" booster with 15" Vac should produce about 377lb of extra input force.
Total force = 977lb
Area of 1" MC = .785 sq in
Line pressure should be about 1245 psi

Fluid Volume and Travel
Front caliper piston area = 5.15 sq in
Rear caliper piston area = 1.77 sq in
Fluid volume for 1" of stroke on the MC = .785 cubic in per MC piston
Front piston travel per 1" of stroke = .0762 in
Rear piston travel per 1" of stroke = .2218 in

I belive the MC stroke to be 1.25", but I'm not certain.

[Apogee]

That helps. I'd suggest that you run a 1" bore MC given the specs above, which you're already running. Assuming you've confirmed that you don't have any air in the MC or system first, my next best guess would be that you need to replace your MC or rebuild it. It sounds like it could be internally bypassing fluid through the first part of the piston stroke.

Tobin
KORE3

[dbranger94]

I was wondering if I should change to a 95-97 Explorer MC? The difference is that 95 and newer Explorers have 4-wheel disc brakes, and as such they have the proper springs inside the MC. The downside is that they have an 1-1/16" MC bore. This will drop my line pressure by about 11.4%, or 142psi using the above figures.

Edit: Another idea is to create a hybrid MC using the Explorer springs and a Ranger MC.

[dbranger94]

I removed the RABS valve last night and took it apart...


Inside the isolation valve solenoid bore.


Inside the dump valve solenoid bore. You can also see the accumulator piston, which is covered in junk.


Solenoids and pressure switch. Seals and o-rings are starting to deteriorate. I'm not even sure if these things could move if they wanted to.


Coils and covers.


Black sludge that is covering the inside of the valve.


Inside the accumulator piston bore. You can clearly see all the junk that is littering the inside of the valve.


Accumulator piston and more junk. You can also kinda see the cracks in the o-ring. This piston was completely seized in the bore and I had to pry it out with a flat screwdriver.

I'm amazed after seeing this that I was able to get any fluid at all to the rear brakes. I'm almost positive this is my problem and why I have no rear brake pressure. Looks like this one will be a core.