Failed go-kart. Brakes lock up on start?

[DAVID_T]

Downer day. I tried to drive my car as a go kart for the first time, and the car rolls whenever it's in neutral and the car is off and the brakes work properly, once the car is turned over the brake system locks up. All 4 brakes go tight. The emergency brake is off, and the power steering works without a hitch.

The brake system is a rebuilt hydroboost for a 96 Cobra Mustang, to a forward/rear brake proportioning valve, to the wheels. I messed with the proportioning valve.

Any thoughts?

[edwardb]

First thing I'd do is double check the proper plumbing. Second thing would be the proper length on the pushrod. It's supposed to have a certain amount of clearance when at rest. That's commonly a problem. Was on the only hydroboost I've done. Doubt anything with the proportioning valve if all four are locking up.

[DAVID_T]

Here's my picture of the unit. I think they plumbing is good, I followed one of those charts made for the FFCars group years ago.

I did some quick checks, when I start the car, the brake pedal depresses completely, and then takes a minute or two to return to start position one the vehicle is shut off.

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[PhilO]

Double check your hose routing. If the pedal depresses when the engine starts, you are getting pressure in the wrong place. If everything checks out, I would think a bad hydro boost and pressure is bleeding by the seal.

Where is the low pressure going to? If you have pressure on the line I would think what you are seeing would happen. Verify all connections on your pump and reservoir are correct.

[DAVID_T]

Ok. I'm on my phone and headed to a workout, so I'm doing this yeah quick.
Color to color for each photo and I'm basing it off of the included diagram. The low pressure fitting in blue goes to a T fitting that combines the the hoses. Again these are from a 96 Cobra, except the Hydroboost was a rebuilt from Cardone.
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[PhilO]

From those photos it certainly looks correct. Does your power steering work? Only two thoughts come to mind. Verify power steering pump is rotating in the proper direction, most are clockwise? Not even sure it would build pressure backwards, but if they do, it would be putting pressure on the low side, maybe you got a reverse rotation pump by mistake? But I would think your power steering would not be working if that was the case. Otherwise I would pull the low pressure hose from the hydroboost off the tee or at the brake and see if fluid flows when the pump is running. Just be sure you are ready to catch fluid wherever you disconnect. There should be little to no fluid from the hydroboost when the brake is not depressed. Only other option would be to just change out the hydroboost.

[edwardb]

So far I'm the only one that mentioned it... but check your pushrod length. Assuming everything else is right (plumbing, hydroboost unit working properly, etc.) that's the number one culprit IMO. Matches your symptoms exactly if the rod is too long, e.g. could apply your brakes without pressing the pedal. I had hydroboost in my Mk4 Roadster build #7750 that came from Forte. The pushrod between the hydroboost unit and the dual MC was unadjusted (too long). I discovered the problem when it wouldn't bleed properly. But had I proceeded, it likely would have done the same thing, e.g. actuate the brakes because it was pushing the MC piston when mounted onto the booster. You should have around .020" clearance with both at rest. This video shows what I'm talking about. Although doesn't give the solution. Can be addressed with spacers or adjusting the length of the pushrod. Or a combination of both. https://www.youtube.com/watch?v=UgZlKN97e8A&t=118s&ab_channel=BuildaCobra.com. At the very least, suggest you check this.

[CraigS]

I agree w/ edwardb. Long ago at a Lexus dealer I worked on a couple of cars that had an almost identical system. Even replacing a master cylinder w/ a new factory replacement we had to go through the measurement process. It was a pain even w/ the factory tools. If the pushrod is too long the car would have the same problem as the OP has. If it's too short you end up w/ a ton of freeplay in the brake pedal.

[rich grsc]

The routing is correct, I think edwardb is onto the problem, that or the hydroboost is bad. Get rid of the brake proportioning valve.

[DAVID_T]

Update:
The power steering works perfectly. I can turn the car with ease when she's on.
I took the MC off and measured, the length of the rod extending beyond the HB and the length of the insert in the MC are both about 1.5-1.6
MC insert was 1.598
Rod was 1.550

I added a .768 size shim that came from FF, no change. I added both, and no change. I took the MC off and started the car, the pushrod goes all the way out. And the brake pedal depresses.

If my plumbing is correct, this has to be a bad hydroboost.

Pretty annoying. I guess it's off to the shops to find one.... ��

[Norm B]

Make sure your low pressure (return line) is not blocked. Try disconnecting the brake pedal from the hydro boost pushrod and see if the rod goes in when you start the engine. Here is a write up I found on hydro boost brakes. Maybe it will give you some ideas. https://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/

Norm

[rich grsc]

Did you check the pedal pushrod length going into the HB, use the spacer between the firewall and the HB?

[DAVID_T]

Hey Rich, yes I have the FF spacer on that part. I don't think I can do anything about the length of that pushrod, it has specialized ends on both sides.

[cob427sc]

I would agree that the pushrod length maybe the culprit. Another way to test it would be to remove the pushrod and then start up the engine to see if the brakes still lock up. I'm not familiar with your entire setup but if the pushrod cannot easily be shortened, what about adding spacers to push the master cylinder further away from the firewall in effect, lengthening the deistance between the pedal and the m/c?

[DAVID_T]

Yeah, I could put one of these spacers between the HB and the firewall, and see if that helps.
The more I am looking at it, the more it is looking like something internal on the HB is acting up.

I'll post with some more findings, later. Thank you all!

[BadAsp427]

Ok. I'm on my phone and headed to a workout, so I'm doing this yeah quick.
Color to color for each photo and I'm basing it off of the included diagram. The low pressure fitting in blue goes to a T fitting that combines the the hoses. Again these are from a 96 Cobra, except the Hydroboost was a rebuilt from Cardone.
159665159666159667
159664

Looking at your first photo and then the diagram, if you are trying to match colors to the diagram then to me you have your two high pressure hoses connected backwards. You show the Pink circle on the bottom, but the red connection in the diagram it should be going to the top. Then the green circle on your 1st photo is the opposite from the diagram also. I do not know anything about the hydroboost, but based on your provided photos, I see the lines hooked up incorrectly.

[JIMLAND]

I have HB and compared your plumbing with mine and it matches so I think you either have a bad HB or pushrod length problem. Add some temporary spacers between the firewall and HB to see if it is a pushrod problem.