I'm new here an posted this in the super duty section. I've had 2 new hydroboosts in the last week PS pump is @ 3yrs old. I can finally move my 35" tires at a stop but if I'm going @50+ I loose brakes pedal gets hard. Could my master cylinder be gone? I've never had this before and everything I've read goes to pump n hydro. Only difference this time is my hydro boost when pressing the brake is loud n pulsated when I move the wheel.

 

What else has been replaced recently and is there an ABS light?

 

Yes abs light is on

 

There is a bleeding procedure in the Excursion forums that will eliminate the problem if there is air in the lines. It seems to be better than the gravity bleed.

Do you have 4 wheel ABS or just rear? Both of those will have a sensor that is located on top of the rear differential and obviously if you have 4 wheel ABS there will be a sensor on each front hub unit. Any of these could be the culprit. AE is one of the best diagnostic tools for isolating which, if any, of the sensors is going bad.

At a minimum pull the sensor out of the rear end, clean it off, and replace it to see if that clears the light. You can try pulling the front ones and doing the same.

If you are sitting somewhere in gear with the engine running with the brakes applied using the same pressure constantly, does the truck eventually start to roll?

Lastly, if you are having this intermittently, drive carefully after each attempt at fixing it. It can rear its head again at the worst possible time.

Removing the 60A fuse under the hood will restore your brakes to a semi-normal state minus ABS until you can figure this out. However, the ABS light will remain illuminated.

 

Change the master cyclinder. Rest is new. Sounds like air in the system but hard pedal and no brakes, that 's what I'd do.

 

I've removed cleaned and put in new rear abs sensor still on going to do the fronts here soon. As for holding the brake pedal n rolling..it did it when I first reinstalled the hydroboost so I attributed that to air.

 

So you think the master cylinder is the cause of the hard pedal n no brakes? It was my next step I just am unsure on how out of the blue its bad when my hydroboost went the actuator blew off

 

Even though they are separate I would always change both. You went threw two hydro boosts, one PS pump, what's left. I have seen master cyclinders do this. But if not what's left, lines. Bleed the hydro boost it easy then change it out. Its cheap enough and now its all new.

 

I'll give it a try and post results thanks

 

One of the reasons that I suggested pulling the 60A fuse was so that you could possibly make a determination as to whether it is mechanically related or electrical.

If you pull the 60A and your brakes are normal, minus the anti-locking feature then, maybe that points to a sensor/computer related issue.

I could be over simplifying things however it will give us more info for when the ABS experts, which I am not, see the post.

 

So if I pull the fuse. My abs light will be on. Brake "should" be normal but just won't have abs correct? I just never thought the abs acting up would make my brake hard as a rock

 

OK I pulled abs fuse. The brakes got hard in drive an at an idle the truck kept going pedal was hard like it did going 55mph

 

OK. So, I am at a loss here. If all else was normal, by pulling the fuse, you should have normal brakes just without anit-lock function and YES, the ABS light will remain illuminated with the fuse pulled.

 

Ya me too. My brakes worked in the driveway I pulled the fuse an my brake went hard an would not stop I went back n forthba few times and brake pedal was hard when I put it in drive foot on the brake truck kept going

 

I'm thinking its a coinsidence and my master cylinder is the problem I have no clue what else