Spongy Brakes

So I lost a wheel cylinder a few weeks back and replaced both rear shoes and drums since I had everything apart for the wheel cylinder. Well after using a scan tool two separate times to cycle the ABS module I still have a spongy brake pedal. I have also manually bled the brakes from furthest to closest. I hear a "whoosh" sound from the booster when pressing on the pedal but still dont seem to have much brakes until about 60-70% of the pedal. I am no losing any fluid and the brake pedal stays the same so I'm leaning towards potentially a bad master cylinder. I'm completely stumped on what the issue may be and both local dealerships are an absolute joke. prior to losing the wheel cylinder I had roughly 1-2" of pedal then good firm brakes and now is seems like I almost have to go to the floor to get anything. It may be in my head but it feels like I build up pedal firmness if I pump the pedal a few times while using them to stop the truck.


Any help is appreciated, thanks.

 

Did you Properly adjust the rear brakes when you worked on them?

Adjust tight until the drum locks then back it off 15 notches.

 

I believe I did but will verify with supplied steps, thanks.

 

I'd take a look at the cylinder more but Jimmyh may have the solution. Keep us posted as I feel my brakes have gotten softer in the last month or so. I've checked for both air in the lines and the level of brake fluid with no luck.

 

Diagnostic steps are key. Spongy is a light to mild pressure that progresses to a firm pressure. That is air. Long pedal travel is virtually no pressure for a while followed by immediate firm pressure. Feels like the pedal is doing nothing till it hits the spot. That is too much shoe clearance. Proportioning valve keeps sending all the fluid to the rears until they resist, then equalizes.

 

Well I dropped both rear wheels yesterday and the shoes were definitely not adjusted correctly. Thanks for all the help, I now have my pedal back.

 

There is no Proportioning valve...

 

Pretty sure the proportioning valve is integral to the ABS valve body. almost impossible to get a disc/drum brake system to be balanced, front to rear, without one. The disc takes 3-4 times more fluid to move the piston.

 

You are correct that the ABS System controls the balance between the front and rear brakes.

A dynamic rear proportioning system is integrated with an existing vehicle anti-lock braking system (ABS) and performs the function of the rear brake pressure proportioning where ABS is not required. The rear brake hydraulic channel(s) are isolated from the master cylinder by activating the rear isolation valves, provided in the ABS, to provide the optimum brake force balance, regardless of vehicle loading, without the use of a load sensing mechanism. The system estimates vehicle and wheel deceleration, vehicle speed, and lateral acceleration as well as the rear wheel slip to dynamically control the rear brake force. By continually updating these control parameters, the system can further increase or decrease the rear brake pressure to maintain the optimum brake force balance throughout the braking maneuver.

 

Watching…
I’ve been battling this issue and have just given up.

 

Thanks for the detailed explanation. Clearly you understand the details involved. This helps me understand why modern cars can keep drum brakes. Now using technology to balance them for an optimal performance. The old setups with Proportional valves were unimpressive and often left stopping power on the table.