Good day
I got a 89 ford f-150 that usually has great brakes they key word is usually. every now and then I would say about 2 to 4 times a day you hit the petal and it goes straight to the floor no brakes. I hit the petal a second time and I have brakes. I noticed that when the petal goes to the floor if I push really hard I can make the back tires lock up but with no front brakes it just slides. I put new front pads on because they were shot and put a new master cylinder on thinking it was going bad but I am still having this problem. I am at my witts end I do not know what to do next

 

Did you check for leaks in the VAC system yet? mainly, the VAC hose coming form the MC.

 

yes I did check the vacuum line running to the booster and I found no leaks

 

This is going to sound like a stupid question, but did you bleed the brakes after you replaced the master cylinder? And if so, in what order? Because if the master cylinder is new, you've either got air in the lines or there's a leak. Next time you go out after it's been sitting for a few hours [overnight would be best], check under the truck for any fresh oil spots near the tires.

If no leaks are obvious in the brake system, try bleeding the brakes, even if you've already done it. And start with the wheel farthest from the master cylinder and work in. So since the master cylinder is mounted on the left side of the engine compartment, you'll want to start with the right rear, then left rear, then right front, then left front. Sometimes air can get trapped in the proportioning valves and that can be a real PITA to get out.

I recommend you have someone pump the brake pedal to do the bleed. Vac pumps are okay but after swapping a master cylinder, it's my experience that they don't work very well for it. Also, have the pedal pumper pump continuously until you see air blowing out of the line, then have them pump 3 to 4 times and then hold the pedal until you tighten the bleeder valve.

And be patient with it, too- just because you don't see any air come out of the line with the first few pumps of the pedal doesn't mean there isn't any to bleed out. You'll also want to have a couple of large bottles of brake fluid handy because it can eat up a lot of fluid.

Give that a try and let us know.

 

When i worked in a shop we had a stake body87- 96 f series come in that needed a master cylinder changed. Me and the boss bled the new MC on a bench and then bled everything when installed? Good luck please let us know how you make out, no brakes is no joke....

 

I am an ASE certified Ford mechanic and I also have had the same problems with my Lincoln navigator I also was puzzled. I replace my master cylinder and the brake booster with no solution in sight I did a little digging and found the cure to my soft pedal problem. It was intermittent and really only happen when I was going slow the issue lies within the ABS wheel speed sensors now the wheel speed sensors are responsible for traction control and breaking when my speed sensors went out I had no idea It was the speed sensors because my brake pedal would go to the floor intermittently and it wouldn’t stop and then when I pumped it you know once or twice I could get it to work again and so I naturally thought it was the brake booster and master cylinder. But once I replace my wheel speed sensors the problem went away. This is all because the computer thinks that the car is in a slide motion or in a loss of traction because the speed sensors are sending incorrect data to the computer so it’s not wanting it to break because of the fact that it’s in a slide and a safety mechanism tells the computer to disengage the brakes and not to brake during a slide or loss of Traction. I would replace these ABS sensors and see where it takes you I fixed my problem

 

The OBS Fords are Rear Anti-lock Brake Systems-one sensor on the diff that also gives its signal to the PSOM. Experience says if the RABS doesn't activate in a hard stop, you can lock the rear wheels. typically, just cruising around, regular stops, the RABS isn't activated anyway. If you make a hard stop and the RABS unlocks the rear brakes which is not common, despite what has been said about them, you still have full front brakes, just a bit lower to the floor. Zero brakes or what seems like zero brakes may well be excessive runout on the front rotors, pushing the pads further away from the rotors than allowed by factory specs while driving. That first brake application that feels dead is needed to get the pads back out where they belong, against the rotors then the next brake application actually makes the pads grip.

If this truck has RABS, then the proper bleeding order is Right Rear first, Left Rear next, RABS, Right Front then Left Front. It's better just to pressure bleed the system to be sure it's bled correctly. Far less chance of introducing air into the system while bleeding. this was how we approached the problem when I wore an ASE Master Technician patch on my sleeve.

Just my $0.02 USD worth. YMMV. One coupon to a customer. Void where prohibited.

 

10 year old thread.

 

I saw that. Still useful information.