Spongy pedal, bad booster?

[ledslinger29]

Hey ya'll
My 68 has a front disc conversion, a brass prop valve, and a vacuum booster. A little while back, my brake light on the dash came on and my brakes weren't stopping right. Previously, I had a nice hard brake pedal, which depressed a little when I cranked the engine. Now it doesn't seem to matter if the engine is running or not, the pedal is spongy. It'll stop the truck, but the pedal goes all the way to the floor. I replaced the prop valve, and all of the lines, bled them several times, with no improvement. So my big question is, how do I know if my booster is bad? I just got a new master cylinder and bench bled, no more bubbles come out when I depress the plunger. i mounted the master cylinder on the booster, with the outlets still plugged, and I still can't get a hard pedal. What do you think?

 

[Apogee]

Spongy pedal typically means you have air in the system, whereas a dead booster would result in a high/hard pedal and low brake torque.

I would back up a step and disconnect the hard lines from the master cylinder and install some inverted flare plugs into the outlet ports. Apply the brakes and the pedal should be high and immediately hard after about 1/2" to 3/4" of pedal travel (unless you have a lot of slop in the pedal pivots, in which case it could be more). At this point, your MC is hydraulically locked. If it's spongy and continues to travel further, then there is still air in the MC, so do a plugged port bench bleed and then reinstall (or do it on the truck). With the plugged port method, you can apply force to the MC firmly once bleed, and if it sinks over time, then the MC is internally bypassing fluid and needs to be rebuilt or replaced.

Once you have a functional MC installed, then connect and bleed out one brake circuit at a time. I usually do the rear first, but it doesn't really matter which you choose. This minimizes the number of variables and makes troubleshooting easier if you're trying to find a faulty component.

The fact that you've been triggering the brake balance warning light would indicate an issue upstream from the combination valve, which would be the master cylinder.

Tobin

 

[ledslinger29]

Thanks,
The booster had an adjustable push rod in it and I need to move it out about 3/8ths of an inch, now I'm getting a little bit of air in with every couple of brake pedal depressions, with the outlets plugged. I'll keep working it... and I didn't think about hooking up one outlet at a time for bleeding, thats a good idea

 

[SS396]

I've had a similar issue on my 75 with drum/drum. After replacing the master cylinder I found my pedal going to the floor on the first pump but firm on the second pump. Was able to resolve the issue by installing an inline residual pressure valve. Not sure if the master cylinder was defective, or if parts store replacements just don't include the proper internal residual pressure valves for the drums.

Does the pedal firm up or is it spongy after repetitive pushes?

 

[taipeichris]

Apogee, aka Tobin,

Excellent post and description on how to trouble shoot the mushy pedal problem! I too have a spongy or mushy pedal. The last time I took my 66 out on 02/29/2020 to a couple of car shows my brake pedal got super spongy and I almost couldn't get the old girl to slow down. Yep, that was a slow surface street drive home.

Happy Saturday!

 

[ledslinger29]

Just wanted to bring some closure to this issue, Replaced the master cylinder, hooked the lines up and had my seven year old help me bleed. Old truck stops like its supposed to now, thanks for the tips y’all.

Sent from my iPhone using Tapatalk

 

[Apogee]

Good brakes = driving confidence...good deal. Glad you got it sorted.