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Losing brake pressure

1973 F-100 4x4

'77 F-250 Custom 4x4
I have a 95 F250 4x4 that I've been working on resurrecting for the past 10 months.
I have replaced the master cylinder, all of the brake lines (steel and rubber), the rear wheel cylinders, rear shoes and hardware, and front pads.

I have bled the entire system twice now, and cannot figure out why I only have brake pressure with the engine not running.

I have looked over the entire system for fluid leaks, and there are none.

When I have the engine running, I can easily push the brake pedal to the floor.
I have noticed that the idle gets a little erratic when I push the brake pedal though.

Would a bad brake booster cause this?

Any ideas?
 

Fellro

Moderator
Staff member
Even if the booster was bad, you should have pressure, they would just feel more stiff. Does it have Rear ABS? The 92 unit went bad and was relieving the pressure to the rears all the time. I bypassed the unit and now the brakes work fine.
 

1973 F-100 4x4

'77 F-250 Custom 4x4
Even if the booster was bad, you should have pressure, they would just feel more stiff. Does it have Rear ABS? The 92 unit went bad and was relieving the pressure to the rears all the time. I bypassed the unit and now the brakes work fine.

Yes it does, I'll have to try that.
It wouldn't surprise me if it is bad, especially since this truck was a plow truck, and it sat for 6 years.

Do you know where is the ABS unit located, by chance?

Also, how do I bypass it?
 
Last edited:

Fellro

Moderator
Staff member
If you have rear ABS, it is likely located almost directly below the firewall on the inside of the frame rail. To bypass, you just take the brake line and connect it without connecting to the abs unit. There is only one line in and one line out.
 

1973 F-100 4x4

'77 F-250 Custom 4x4
If you have rear ABS, it is likely located almost directly below the firewall on the inside of the frame rail. To bypass, you just take the brake line and connect it without connecting to the abs unit. There is only one line in and one line out.

I'll have to do that, as soon as the weather lets up and it gets above 20 degrees.

Thanks for the info :)
 

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