Brake's in my daughters Explorer Intermittent weirdness. Need a 2nd opinion or three

My daughter has a 94 explorer. She recently had the EGR sensor control unit replace (she got hosed on the price but that is another story)

I looked at the replacement part and it is not a direct factory replacement it Si a slightly different unit that the mechanics has to cut the harness and install the new one with a new plug. I looked up the parts and there are two types of control units listed for the vehicle with different plugs. I also found the old one that they took out sitting on top of the motor.

Anyway skip forward and she started getting an intermittent brake failure issue after she has the EGR module replaced. The symptoms are

Peddle goes to the floor and immediately on the 2nd pump of he brakes they are rock hard (like there is not power assist)

My suspicion is that the EGR control unit is wrong or wired backwards. this would actually pull on the power assist causing a gap in the power assist causing it to go to the floor the first time and then since the module is either wired backwards or is correct it is diverting vacuum away from the power assist and thus on the 2nd pump you get rock hard brakes with out any power assist.

This is an intermittent problem. It occurs enough so the car has been pared until we get it sorted. I have got an OE replacement EGR vacuum control unit from the local P&P with the correct plug etc and I am going to swap that in. The next steps I see are replacement of both the master cylinder and the brake booster (again I would probably just get a complete unit from the local P&P unless they are cheep enough new.

I just hair to start tossing parts at it and being a teenager her budget is not unlimited.

There is no apparent leaking of brake fluid anywhere (from the lines or the master or booster). The pads look good all around.

Anyone got any other ideas?

If the pedal goes to the floor without the vehicle making a very sudden stop I would tend to think that it has nothing to do with the power assist. My guess is one of the seals in the master cylinder is buggered up and the brake fluid is just going around the seal when you push on the brake pedal. Not sure why it would be rock hard on the next application though - maybe the seal or the piston is rocking back and forth and getting hung up on something.

Bob

In reply to Schmidlap:

could it be releasing all the vacuum assist stored in the booster in that one trip to the floor, making the next press on the pedal hard until the vacuum builds up again?

Strizzo wrote: In reply to Schmidlap: could it be releasing all the vacuum assist stored in the booster in that one trip to the floor, making the next press on the pedal hard until the vacuum builds up again?

Nope. If it goes to the floor without any brake actuation, it's a hydraulic issue.

I presume its the egr pressure sensor that failed, since every Ford ever built has has several changed. The lines that go to the EGR pressure sensor have nothing at all to do with engine vacuum. They sense the pressure differential from the two sides of the EGR valve.

Your master cylinder has failed.

Is your name Everett Sinchu?

I am not sure this is related, however my 98 Explorer was exhibiting some funky brake issues lately. It would almost go to the floor on hard stops yet was firm on regular stops. It also exhibited irregular wear on the inboard pad on the front right side.

What it turned out to be is that I needed to replace both front calipers. They are a 2 piston design and if both pistons are not pushed in together the piston rock in the bore and wear funny. This leads to them not balancing the forces properly. When you got on the brakes hard, the pressure would force all the brakes to move, but when you were braking lightly the RF brake was carrying most of the load, as the pad was not retracting. (This also explains the loss of gas mileage that I was having). The pins were well lubed and everything, but the uneven pressure would not allow the caliper to float properly.

Since I did both calipers - good pedal all the time. Calipers were about $32 each from Rock Auto.

Might want to check this.