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Brakes on a 1997 e-150 van

i need help with my e-150 customized van brakes. i have replaced the front rotors, callipers, brake pads, rear drums, all hardware, brake shoes (rear), rear wheel cylinders,two master cylinders,and have had the system bled 4 times.my problem is the brakes work the way they should most of the time.every so often when you push down the pedal it will start off working right then the pedal gets hard and will not go down any farther. the van will not stop. if i let up on the pedal and push down the pedal it will stop the van the way it should.what else can i do ? i have heard that it could be a bad rear actuater valve? i am at a loss on what to do next. help!!!

My first look would be at the check valve for the vacuum booster - it seems possible that on occasion your booster is losing all of its vacuum and you’re losing the power assist. The valves are normally cheap so I’d probably just put a new one in. Other than that it may be a problem with the booster diaphragm itself - not so cheap and easy. But anyway…the booster is where I’d look next.

I have seen Ford products with vacuum booster problems and related check valve problems and yours does seem to be just like the others.

Some E150s come with a hydroboost brake booster. Check to see if you have this unit in your van. The power steering pump provides the power assist for this unit. If so the accessory drive belt may be slipping over the pump pulley when braking demand is high. This would cause diminution of the assist which is what you are describing.

If you do have the vacuum assist brake booster, the problem is probably a bad brake booster unit. It probably won’t be the check valve because the only time the check valve is a problem is when it doesn’t pass air i.e. low or no vacuum in the booster or when it remains open i.e. loss of assist when the engine dies.

Hope that helps.