Lazy Daze Owners' Group

Brake pedal "thump" when starting

August 07, 2014, 07:36:42 pm

Yahoo Message Number: 147855

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My SOB uses the 2004 E450 chassis, same at the LD of that vintage.
Sometimes when I start it the first time for the day, I feel a "thump" in the brake pedal just as the engine starts. Later starts - no thump.
The brakes and everything else seem to work properly. Does anyone know what that "thump" is, and if it's a symptom of a problem?

Eric Greenwell

Re: Brake pedal "thump" when starting

Reply #2 – August 09, 2014, 02:54:09 am

Yahoo Message Number: 147877

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"I wonder if it is the hydro-boost rebuilding pressure that had bled off while the rig has sat for a while."

That's exactly what is happening. The hydro-boost assembly contains an accumulator which stores pressurized hydraulic fluid for use to brake the vehicle if the engine should stall. When new, a hydro-boost will hold pressure for days, even weeks. But after years of use it's check valves may not seal as effectively and leak down faster.

A test to see if the brake unit is pressurizing requires pumping the brake pedal several times, with engine off, until the pedal is rock hard.  Then, while holding your foot firmly on the pedal, start the engine. You should hear and feel a good thump in the pedal when the power steering pump reenergizes the hydro-boost.

Larry

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