NOT the master cylinder: brake pedal going to the floor story and solution, 2 part problem.

[sonyslave]

This was on an '03 2wd 

 

The brakes suddenly were not as effective.  Had to stand on the brakes and even then, much reduced braking force.  With the engine off the pedal would sink to the all the way to the floor. 

I thought the master cylinder went out since there was no hydraulic resistance felt at the pedal with the engine off.

 

First: I inspected for leaks and found one rear wheel cylinder had a small leak. The exterior of the drum housing on the side facing the pumpkin had a barely discernable, small  damp spot in the grime near the bleeder nipple.

That wheel cylinder was bad, fluid weeping past the rubber seal, clearly visible when lifting the outside lip of the rubber seal.

I swapped in a new wheel cylinder (fyi you don't need to take apart the brake shoes, just crank open the adjuster.  After install is complete you have to crank it back closed while holding open the one way stopper arm)

I power bled that line at the wheel cylinder -it was strange because it was difficult for the power bleeder to get a lot of fluid out of the line -but eventually enough came out that I considered it to be air bubble free.

 

The pedal was still sinking to the floor...

I slightly cracked  each line (2 lines)  at the master cylinder connection (while pedal was being depressed)  to see if there was pressure passing out of the MC, there was. So I assumed the MC was good.

 

I power bled more at all 4 corners TWICE pumped brakes with engine of and on, and still, not a lot of fluid was coming out, but enough that I thought maybe there are no air bubbles.

 

Next I pumped pedal with the engine off, the pedal was firm, a good sign that the system was holding pressure and bubble free.

Next I test drove and noticed an air hiss under braking, this is a telltale sign of a leaking brake booster.

Next I swapped in a new brake booster and all is well.

 

Theory: the rear wheel cylinder failed and allowed air into the line creating the sinking to the floor brake pedal.  Then: maybe full extension of the brake booster under repeated 'pedal to the floor' strokes caused the brake booster diaphragm to tear?

 

Takeaway: the master cylinder is fine despite the symptoms being indicative of a failed MC.

 

Also: this is the 2nd replacement brake booster for this vehicle (master cylinder showed no leaks).  1st replacement BB was from O'Reilly's, a remanufactured unit from China bought in 2019, and was garbage, it start having a rubber squeak sound 2 weeks after install, and failed at 5 years.  Upside, lifetime warranty on the original purchase, 2nd BB was no charge (reman unit for Mexico) -but not sure if the lifetime warranty applies to the replacement part.

 

If you diy this have various extra ratchet extensions and wobblers on had, also deep well sockets.  Working under the dash is a bitch so allow extra time.

 

 

Edited October 21, 2024 by sonyslave

[Slartibartfast]

Good work tracking it down! I hate working on brakes.

 

I would not be surprised if the booster was the whole failure, and the wheel cylinder just happened to be on its way out at the same time. Hopefully the new booster outlasts the last one.

[sonyslave]

Thanks!  It took a while to figure out. I have not done much work on brakes, however hydraulic systems are pretty straightforward if one considers a master cylinder to be basically a syringe with 2 exits.   

-Also just fyi (don't want to give anyone the wrong assumption) when the brake pedal goes all the way to the floor that is not usually symptom of brake booster failure, typically it could indicate a master cylinder seal failure.

[FirstGenFreak]

Booster function won't affect your pedal, in fact a failed booster will give you a rock hard pedal. 

Sounds like a strange one.