Back­flow Brak­ing

Jp Magazine - - Mailbag -

I’m going to chal­lenge your brake resid­ual pres­sure valve ex­pla­na­tion (Your Jeep, May ’18) re­gard­ing the small resid­ual pres­sure in the brake lines of drum brakes. I’m sure we were told in school in my ap­pren­tice days 50-plus years ago that pres­sure was to keep the cylin­der cups suf­fi­ciently seated so as not to leak. The star wheel ad­juster keeps the shoes up close to the drum. The re­turn springs are strong enough to over­come that min­i­mal line pres­sure. That’s my story and I’m stick­ing to it.

Love your tech stuff, even as a nit­picker.

Clarke Lacey Van­cou­ver Is­land, Bri­tish Columbia

You’re right! While it is true that the drum brake shoes are held in place close to the drum brak­ing sur­face with the ad­juster as­sem­blies, the wheel cylin­der pis­tons and plungers can still re­tract away from the shoes. The resid­ual pres­sure valve in a drum brake sys­tem is used to keep the wheel cylin­der pis­tons from work­ing their way back into their bores when the brake pedal is re­leased. A small amount of line pres­sure (around 10 psi) al­lows the wheel cylin­der plungers to main­tain con­tact against the shoes in the drums. This pro­vides much quicker drum brake en­gage­ment when the brakes are ap­plied and re­duces brake pedal travel.

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