I have a 1989 Ford 150 Standard Cab L6. I have been experiencing brake problems there is no brake pedal pressure. In my troubleshooting efforts, I replaced the front calipers and bled all the brakes and this restored the brake pedal pressure while sitting in the driveway. After taking the truck for a test drive around the neighborhood, the brake pedal would not stay built up. The problem is when traveling 25 MPH and over the brakes work fine and slows the truck down, but when the truck reaches 20 MPH the brake pedal goes straight to the floor thus resulting in no brakes to stop the truck.

What should I do next in troubleshooting the cause of the no brakes at low speed? Do I need to bled the brakes again? Or is the master cylinder bad and in need of being replaced? Or is it the ABS system? This is an everyday truck, which I need to get back on the road and be confident it will stop when I apply the brakes.

Any and all help in this matter is greatly appreciated.

moving in Slow Motion!

 

Regardless of the speed the brakes should work the same. Start the truck and press the brake pedal, It should be firm. If its not them you must re bleed the brakes if its firm on the first push of the pedal continue to push and if you feel the pressure gradually fading away the master cylinder is bad. the different from when you just pushing the pedal in your driveway with the engine not running is that the power booster is not giving you any boost. it may feel great but as soon as you start the engine it finds any weak "air in the line" point if there is any.

My guess is that you need to re bleed the brakes.. you may have to bleed each side 10 times. but remember if it takes more than one pump to have brake pressure its not safe.

 

Bleed the rears.

 

if bleeding the brakes doesn't work then you may have a problem with the brake booster itself. I had a '91 civic a few years ago with over 308,000 mi on it and the pedal would fall to the floor until you pumped it a few times and the pressure would build up and then fall off again. The plunger in the brake booster went bad and kept letting the pressure out. It was a pretty easy fix once I figured that out.

 

Sounds like a bad master cylinder (MS) unbolt it from the booster and check for leaks, there is enough give in the brake line to let you get a look at the piston area inside.

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I was gonna say, after you bleed all 4 corners check the brake lines, the make sure the Master Cylinder is not leaking. Then go on to check the booster.

 

Simple test of a brake booster that all should remember. With the engine off, pump the brakes, pedal should get hard and seem to be higher, hold pedal down and start engine. When the booster gets vacuum from the engine the pedal will go down to normal height. All brake boosters act this way, thats if they are working.