No front brakes

LDALLEN

MEMBER
1978 INTERNATIONAL SCOUT
4.4L
4 CYL
TURBO
4WD
AUTOMATIC
50,000 MILES

I have adapted the GM hydro-boost to the vehicle and cannot get the front brakes to work. New master cylinder, new brake lines, new calipers. I have eliminated the apportioning valve, tried both ports out of the master cylinder, rear brakes work with normal pedal if one port is blocked. If both front and rear are hooked to master, you can pump them up but when you let the pedal up for a bit then it goes back to the floor. But block one port and the rear brakes work with solid pedal. I have bled from top and pressured up from bottom, cannot think it is air. With engine running power assist works as does power steering. With just the front brakes hooked up with one port blocked, same situation, you can pump up to solid pedal, no leaks, but when you let up, pedal returns to the floor.

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi and thanks for using 2CarPros.com. I ask that you bear with me since it is a modification which has been done and not the OEM set up.

I have to be honest, it sounds like the master cylinder is bad or you still have a tone of air in the system. The master cylinder is a two stage system. One of the reservoirs holds fluid for the front and the other for the rear. To me, it sounds like you have one piston working and the other bypassing. Hence, you can pump it up and the next time, it goes to the floor. I found a picture online which indicates the design and how it works. Take a look at it and let me know your thoughts.

Here is a link which discusses the causes of the brake going to the floor for your review:
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Also, here is a link for bleeding the system:
https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

I hope this helps. Let me know what you find or if you have other questions.

Take care,
Joe

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Saturday, May 26th, 2018 AT 8:19 PM

LDALLEN

MEMBER
3 POSTS

I had decided to try another master and ordered new this time rather than rebuilt. I have a pressure bleeder and am able to apply 30 psi when bleeding up from the bottom which you would think should do the job, although I agree, it acts like the system still contains air. And I have applied that same 30 psi to the master cylinder full of fluid with no visible air coming out at the calipers. I will let you know the results of the new master when it arrives.

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Saturday, May 26th, 2018 AT 10:22 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Sounds like a plan. Let me know how things work out for you.

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Sunday, May 27th, 2018 AT 7:46 PM

LDALLEN

MEMBER
3 POSTS

New master made no difference, but I have solved the problem. No amount of bleeding will remove the air if the bleeder is not at the top of the caliper. I had to remove the caliper and turn it so the bleeder was at the very top then the air came out and reinstalled I had pedal. So the solution was simple, do not know why it took me so long to see it.

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Wednesday, June 6th, 2018 AT 8:56 PM

STEVE W.

MECHANIC
13,573 POSTS

Sounds like you were given the wrong caliper. The bleeder should always be at the top but on many vehicles they use the same casting for both calipers, The only difference is which location they drill for a bleeder. I have even seen some drilled to fit either side.
If the bleeder is down on both sides you just need to remove them both and swap sides.