Do I Need a Bench Bleed?

[floorit426]

Hi,

I pulled the master cylinder and booster, as a unit, over the winter, to paint the engine compartment. I did not bench bleed the mc, before re installing it. We just took the car for a test drive, and the brakes started out, with plenty of pedal. After about 1/2 mile, we started running out of pedal. Did I screw up, by not bench bleeding the mc?

Thanks

[1 Wild R/T]

Doubt it... Look for a leak.. Or the M/C has taken a dump... Pretty common... The normal travel zone is maybe 1.5" of travel... When you reassembled the car & bled the brakes the piston moved to an area where it doesn't normally go... There was probably some crud bonded to the M/C bore which likely damaged the seal....  Seen it many times...   Sometimes you get lucky & can take it apart, clean out the crud & the seal is Okay.... After disassembly you will need to bleed the M/C but I never bench bleed, it's messy... I use a bleeding syringe with the M/C mounted on the car, as soon as I pull the syringe away I connect the brake line... Then do the second port...  Now with a wrench on each fitting & a old towel under the ports/lines have someone depress the pedal & crank the fittings down as fluid is flowing...

https://www.amazon.com/Cardone-10-5000MCB-Master-Cylinder-Bleeder/dp/B002JSM7HA/ref=asc_df_B002JSM7HA/?tag=hyprod-20&linkCode=df0&hvadid=312026024613&hvpos=1o3&hvnetw=g&hvrand=13969022200203055824&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032278&hvtargid=pla-491509626214&psc=1

[70 Challenger Lover]

I assume you bleed the brakes thoroughly at all four wheels after putting the MC back in. I did this exact thing to my RT a couple months ago and I did not bench bleed it. Just dropped it in and bled at all four corners. Still works perfect though I have not had the opportunity to drive it a lot yet.

[floorit426]

Yes, I bled the brakes, and got a normal pedal. Looks like the problem may be a bad master cylinder, then. Right?

[70 Challenger Lover]

That would be my guess.

[1 Wild R/T]

Typically if a seal in the M/C got damaged pressing lightly on the brake pedal it will steadily sink to the floor but sometimes when you push hard on the pedal the seal holds & you get a good pedal