Breaks not breaking

In the past I had an issue with my ABS pump not working correctly. I replaced that and I haven’t had a problem since. I also have a three month old master cylinder. A week ago my breaks weren’t working except one in the passenger rear. I asked a shop their opinion and they said it’s most likely a bad ABS pump. So I’ve changed that out again and the problem still remains. I changed out the 40 and 50 amp fuses as well. Any ideas on what to do next?

 

I think I would verify that there was indeed no fluid pressure getting to the calipers that aren't working first perhaps? If there is pressure at the caliper then check that the caliper(s) aren't frozen.

Alternatively start at the output side of the ABS pump and see if there is pressure at the line. I'm not familiar with the specific systems on the truck but is there a proportioning valve somewhere? That would be another place to use as a check point.

Oh, and breaks, not breaking... from a professional truck braker. There's a joke in there somewhere.

 

Could be air trapped inside ABS module. Start bleeding and engage inlet/exhaust valves in that corner.

 

I’ve bled all four calipers and there’s no air. I’m not sure what you’re referring to by the inlet/exhaust valve though. Could you clarify?

 

The air is (can be) trapped inside an ABS module itself. You won't see any air until ABS valves get opened and let it out.

I was fighting with squishy brake pedal in my old Volvo for years. Bled more than a gallon of fluid, used a vacuum pistol type bleeder and a Motiv pressure bleeder - no nothing. All gone once I figured out how to engage the valves. There was a huge air bubble escaping the brake line, and the brakes were like new since then.

 

Another thing to add:
When I press the break down normally only one break is compressing. I believe it’s the passenger rear. When I hit the breaks harder all four are breaking.

 

Brakes...not breaks. Has the ABS system been scanned with reader?