Brake issue on my 05 Sport 4.0

[LetsFlyMan]

I've never had an issue with my truck til now. Randomly upon startup I'll get a very hard brake pedal with seemingly no pedal pressure. The brakes pretty much don't work.

Then after a few seconds, the brakes return to normal, and my truck stops on a dime.

No dash lights or warning sounds and brake fluid is full.

Do you guys have any idea what the issue could be?

 

[AuSeeker]

I've never had an issue with my truck til now. Randomly upon startup I'll get a very hard brake pedal with seemingly no pedal pressure. The brakes pretty much don't work.

Then after a few seconds, the brakes return to normal, and my truck stops on a dime.

No dash lights or warning sounds and brake fluid is full.

Do you guys have any idea what the issue could be?

It sounds like possibly the accumulator, which is one part of the little electric motor/pump/accumulator assembly on the master cylinder/ABS booster assembly is randomly losing pressure while the vehicle is not running, there's a little O-ring that seals it that could be bad or possibly the accumulator itself.

It could also be either the little electric motor or the pump going bad, you should be able to hear the electric motor running as soon as you turn on the ignition and after about every 3 or so depressions of the brake pedal and should normally run 10 to 15 seconds to build up or rebuild up the pressure in the system.

 

[LetsFlyMan]

Thank you. I'll check that. Makes sense.

 

[SocalSam]

What AU said. Brake booster assembly failure is a stranding event at best. It is critical to resolve ASAP.