Hydroboost Power Brakes - Potential Safety Issue?

[JIMLAND]

It is my understanding with a dual master cylinder, it is designed such that if you have a leak in the front brakes that the rear brakes should still work and like wise, if you have a leak it the rear brakes, the front brakes should still work. Here is a good article that I found that describes the internals of the tandem/dual master cylinder and how they are supposed to work when a leak occurs: http://www.britishv8.org/articles/tandem-master-cylinders.htm

What I'm finding with the hydroboost power brake setup I have in my MK4 is if I have a leak in either the front OR the rear hydraulic brake circuit, then I loose both front and rear brakes. This is the test I ran. I bled both the front and rears and had good pedal pressure and good brakes on all 4 wheels. I had my son apply the brakes (pedal went down about 1/3 of the way) and I went to all 4 wheels (car was on jacks) and confirmed that all 4 wheels had brakes. I then cracked the bleed on the front right caliper and the pedal went to the floor and my son held it there. I then went to all 4 wheels and I could turn all 4 wheels, no brakes front or rear. I repeated this test but this time I cracked the bleed on the rear right caliper and same thing, lost brakes to all 4 wheels.

What I believe is happening is with the hydroboost/master cylinder setup I have, there is not sufficient pedal / push rod travel to cause the master cylinder internal pistons to "bottom out" and allow the good brake circuit to pressure up (see article). I measured the total linear movement of the push rod coming out of the hydroboost that pushes on the master cylinder and it was about 0.9" with full pedal travel. I have the Wilwood brake pedal box and the pedal is as far forward as it will go (almost touching the 3/4" tube) so I have the maximum amount of travel possible. I also confirmed that there is no slop between the pushrod and the master cylinder when the pedal is in the rest position. On the bench, you could push in on the piston of the dual master cylinder and it will move about 1.5" before it bottoms out which seems to indicate to me that I might need closer to a 1.5" travel on the push rod vs the 0.9" that I'm seeing.

So....... has anyone experienced this problem? Does it sound like insufficient pedal travel is causing my problem? Any way to solve this problem?

[JRD56]

Jim, I too have a hydoboost setup on my car. I currently have the body off for painting but did drive the car about 50 miles prior to taking the body off. The brakes seem to work fine. I did plan to bleed the brakes one more time while the body was off so I'll do the same test you did. (I'll even use my son to help). My setup came off a 2000 Mustang GT, but I did fabricate an adapter plate between the pedal Box (2000 Mustang GT) and the hydroboost unit to lessen the angle off the master cylinder. I tried to keep the push rod distance the same as the original.

I'm in the middle of painting my car so it may be a few weeks before I get to it but I'll let you know what I learn from the test.

Jim

[CraigS]

It would be interesting to have an MC that was speced for the same year/model as your hydroboost unit to see if it has less travel than the MC you have. While I have never tested the same as you have, your conclusion sure seems correct to me. I could see the booster (hydro or vacuum) might have .1 inch less travel than the MC to keep from destroying the MC, but having only about 2/3 travel doesn't seem right to me.

[JIMLAND]

JRD56, my brakes work fine as long as the front and rear brake circuits are closed. I just happened to find this out by accident since I started bleeding the front brakes with no fluid in the rear reservoir and could never get braking on the front circuit no matter how much I bled the front circuit. Please let me know what you find when you bleed the brakes.

CraigS, good idea to check on the MC that originally went with the Hydroboost unit I'm using. Agree the hydroboost stroke should be less than the full stroke of the MC.

[canuck1]

Jimland,

Are you using the factory distribution block (the combination shuttle/proportioning valve that is meant to follow the MC output circuits and then direct fluid front or rear)?

Sean

[JRD56]

Jim, I did the final bleed on my brake system today. I did a similar test as you did. I applied the brake pedal, cracked one of the front bleeder valves and confirmed the rear brakes were still locked. I'm using the pedal box, hydroboost unit and master cylinder from a 2000 Mustang GT, so pretty much a stock setup. I do have an after-market proportioning valve in the rear brake line. Hope this helps.

Jim

[JIMLAND]

Jimland,

Are you using the factory distribution block (the combination shuttle/proportioning valve that is meant to follow the MC output circuits and then direct fluid front or rear)?

Sean

Sean,

I'm using the Wilwood Combination Proportioning Valve (http://www.wilwood.com/MasterCylinders/MasterCylinderProd?itemno=260-11179). My understanding of the valve is that it will adjust the rate of increase in the rear line pressure in proportion to the front line pressure. So in the case of bleeding the rear, it should have no effect on the front but what I had was loss of front if I bled the rear and likewise loss of rear if I bled the front. Also, I had the proportioning valve cranked all the way out (fully clockwise) to create the least proportion or the most pressure in the rears. Based on the design of the dual reservoir MC, I believe the problem is that the hydroboost is not stroking the MC piston enough to cause the internal pistons in the MC to bottom out which is what is needed in order to pressure up the remaining good brake circuit.

Jim

[JIMLAND]

Jim, I did the final bleed on my brake system today. I did a similar test as you did. I applied the brake pedal, cracked one of the front bleeder valves and confirmed the rear brakes were still locked. I'm using the pedal box, hydroboost unit and master cylinder from a 2000 Mustang GT, so pretty much a stock setup. I do have an after-market proportioning valve in the rear brake line. Hope this helps.

Jim

Jim,

Thanks for running the test. Since you are using a hydroboost/MC combination that was designed for each other, I would expect it to work as designed or intended. The setup I have is from Forte which I'm told is a 2014 Mustang MC connected to a 97-99 GM Truck hydroboost. I believe this GM hydroboost does not provide enough stroke needed for the 2014 Mustang MC internal piston(s) to bottom out which would then allow the good brake circuit to pressure up as it is designed to do. The GM hydroboost has a 0.9" stroke while the Mustang MC has a 1.5" stroke. I need to get my hands on the GM MC that was designed for this GM hydroboost and check it's stroke. If it's around 0.9" then that matches the 0.9" hydroboost stroke that I have and I believe that would explain the problem I'm seeing.

Jim

[canuck1]

Sean,

I'm using the Wilwood Combination Proportioning Valve (http://www.wilwood.com/MasterCylinders/MasterCylinderProd?itemno=260-11179). My understanding of the valve is that it will adjust the rate of increase in the rear line pressure in proportion to the front line pressure. So in the case of bleeding the rear, it should have no effect on the front but what I had was loss of front if I bled the rear and likewise loss of rear if I bled the front. Also, I had the proportioning valve cranked all the way out (fully clockwise) to create the least proportion or the most pressure in the rears. Based on the design of the dual reservoir MC, I believe the problem is that the hydroboost is not stroking the MC piston enough to cause the internal pistons in the MC to bottom out which is what is needed in order to pressure up the remaining good brake circuit.

Jim

Sounds like you have it figured out. I hadn't seen your build info and so thought this might have been an incorrectly installed or malfunctioning factory combination proportioning/shuttle valve but you've ruled out that longshot in any case.

I'm sure Mike Forte will work with you in coming up with a fix. Good luck!

Sean

[dbo_texas]

Jim,

Thanks for running the test. Since you are using a hydroboost/MC combination that was designed for each other, I would expect it to work as designed or intended. The setup I have is from Forte which I'm told is a 2014 Mustang MC connected to a 97-99 GM Truck hydroboost. I believe this GM hydroboost does not provide enough stroke needed for the 2014 Mustang MC internal piston(s) to bottom out which would then allow the good brake circuit to pressure up as it is designed to do. The GM hydroboost has a 0.9" stroke while the Mustang MC has a 1.5" stroke. I need to get my hands on the GM MC that was designed for this GM hydroboost and check it's stroke. If it's around 0.9" then that matches the 0.9" hydroboost stroke that I have and I believe that would explain the problem I'm seeing.

Jim

Curious to know the outcome of this - did you end up having to swap the MC or was it something else? I believe I have the same setup from Forte - I haven't charged the brakes yet but added this to my list of things to check as soon as I do. If it was something else, please share details.