Dual Braking Systems and Pedal Travel

[George7941]

On every vehicle I have bled brakes on by pumping the brake pedal, the pedal sinks right to the floor when stepped on after cracking open one bleeder valve. Shouldn't the pedal stop halfway since one half of the braking ayatem is still intact?

Makes me wonder what happens if I ever have a problem with the braking system whike I am driving. I suppose pumping the pedal will bring it up somewhat.

 

[John_K]

The one time I blew a brake line the pedal basically hit the floor even though the front brakes were intact. Did seem strange. Fortunately I could still stop just slowly. Pumping did help.

John

 

[mechtech2]

George - That is a good point that I never considered.
I have bled many cars, too!
After pondering this for a minute, it must be the crossover valve that gets pushed to one side or the other.

 

[andyd]

My old Grand Wagoneers have a proportioning valve. I thought this was bias the system to the front brakes and to prevent the rears from locking up. The dual circuit that cars have had since 1967 is a dual piston mastercylinder with front and rear reservoirs. I dont think my BMWs have sny sort of valve. Whenever I had to bleed the brakes after replacing brake lines on the Jeep, I havent had to mess with the p valve. Ditto with the BMW.

 

[George7941]

Mechtech2. I think you are referring to the pressure differential switch. If that is what is causing the low pedal, then pumping the pedal should bring it up halfway.

I should test this theory out next time I bleed brakes.

 

[George7941]

On second thought, if the pressure differential switch (PDS) is causing the low pedal, then it would only cause it for a couple of strokes. After that , we should have the pedal up halfway since the PDS piston will bottom out on one side of the PDS and stop bleeding off pressure.

So, I don't think the PDS is causing the low pedal.