Mushy Spongy Brake Pedal but no apparent vacuum leak

[68 Sport Satellite]

Trouble-shooting increase in mushy spongy pedal - just added vacuum can for additional vacuum reserves to try and firm up pedal, but after adding canister, mushy spongy pedal is worse and pedal goes closer to the floor. Prior to this, I had 2 different shops do extensive bleeding of the lines with the brake booster hose going from brake booster check valve to intake manifold fitting.

Power brakes, Front disc (wilwood) Rear drum (smallest brake cylinder size, NEW).
Raybestos blue 1-1/8" MC
Hose routing is currently intake manifold to vacuum canister check valve, then vacuum canister brass fitting to brake booster check valve.
I inserted plastic strip into brake booster check valve hole to see if the booster had any fluid inside and it was dry.
My cam is not aggressive. It's 475 lift with duration .220/.224 at .050". Just trying out the vacuum can to see if it improves braking.

After adding the vacuum can, I was hoping for a firmer pedal with better bite and no mushy feel at all. Instead, the pedal got mushier and when I press the brakes with car stopped in Park, I can hear the squoosh squoosh sound. I then performed the following brake booster/vacuum can/check valve trouble-shooting that I found online and the car passed all of these tests, giving no clear indication of a vacuum leak or poor vacuum supply. Any ideas on how to improve things?

1. Sit behind the steering wheel, set the transmission to park (automatic) or neutral (manual), set the emergency brakes, and start the engine. Let it idle for two minutes and then shut it off.
2. Pump the brake pedal at normal foot pressure four times and hold your foot on the pedal pressing down slightly on it.
3. Start the engine. As you start the engine, you should feel the brake pedal moving downward slightly, about an inch or less. Otherwise, you don't have enough vacuum in the brake booster. To locate the fault, do the Engine Vacuum and Brake Booster tests described in the following sections.
4. With the engine still idling, remove your foot from the brake pedal and turn off the engine.
5. Depress the brake pedal (using normal foot pressure) four times. If you notice the pedal rising after the second or third you depress it, the booster is more likely holding vacuum. Otherwise, a vacuum leak is affecting booster performance. To locate the problematic area, test engine vacuum and brake booster performance using a vacuum gauge and a handheld vacuum pump as described in the next sections.
6. Start the engine and let it idle.
7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine. The pedal should hold its position. If not, there's a leak in the brake booster, valve, vacuum hose, or intake manifold. Check the vacuum hose, booster check valve, engine vacuum, and brake booster as described in the following sections.

 

[TopBanana72]

What kind of booster do you have? I have a cardone that seems sketchy to me. This sounds master cylinder related too. When I got a Raybestos MC, I had quite a time adjusting the pushrod to get it right. It works OK, but I am going to get my original MC and midland ross booster rebuilt.

 

[68 Sport Satellite]

What kind of booster do you have? I have a cardone that seems sketchy to me. This sounds master cylinder related too. When I got a Raybestos MC, I had quite a time adjusting the pushrod to get it right. It works OK, but I am going to get my original MC and midland ross booster rebuilt.

It's an 8" dual diaphragm from Master Power Brakes.

 

[68 Sport Satellite]

Additional question for all of you - is anyone running a power brake booster for Front disc / Rear Drum that is NOT a dual diaphragm? I've always suspected a possible problem with my aftermarket dual diaphragm booster purchased in 2012 as it's the only item that has remained constant as I've messed with different master cylinder sizes, but all external tests usually check out fine, except for the constant spongy pedal feel. The car still stops, but it is not impressive. I would love to try a single diaphragm OEM booster to compare. Any suggestions for a size that would fit my mild cam 1968 Plymouth Satellite? All of my searches turn up Bendix style dual diaphragm. I would love it if I could adapt a booster from a late model car that would work with my setup.

 

[kiwigtx]

Are you running DOT 3 or DOT 5 fluid?

 

[blue69runner]

Had this happen to me a few times. Did all the checking and chasing. It ended up being a pin hole in the diaphragm. I can agree on the length of the pedal rod it must be the right length also. Back in the day rebuilt my own power booster's. Hope you get it figured out. I am running manual brakes and have plenty of stopping power. Disc. front and drums rear. Replaced the front with the wheel wood disc brakes. Love them.

 

[68 Sport Satellite]

Are you running DOT 3 or DOT 5 fluid?

DOT 3

 

[toolmanmike]

Trouble-shooting increase in mushy spongy pedal - just added vacuum can for additional vacuum reserves to try and firm up pedal, but after adding canister, mushy spongy pedal is worse and pedal goes closer to the floor. Prior to this, I had 2 different shops do extensive bleeding of the lines with the brake booster hose going from brake booster check valve to intake manifold fitting.

Power brakes, Front disc (wilwood) Rear drum (smallest brake cylinder size, NEW).
Raybestos blue 1-1/8" MC
Hose routing is currently intake manifold to vacuum canister check valve, then vacuum canister brass fitting to brake booster check valve.
I inserted plastic strip into brake booster check valve hole to see if the booster had any fluid inside and it was dry.
My cam is not aggressive. It's 475 lift with duration .220/.224 at .050". Just trying out the vacuum can to see if it improves braking.

After adding the vacuum can, I was hoping for a firmer pedal with better bite and no mushy feel at all. Instead, the pedal got mushier and when I press the brakes with car stopped in Park, I can hear the squoosh squoosh sound. I then performed the following brake booster/vacuum can/check valve trouble-shooting that I found online and the car passed all of these tests, giving no clear indication of a vacuum leak or poor vacuum supply. Any ideas on how to improve things?

1. Sit behind the steering wheel, set the transmission to park (automatic) or neutral (manual), set the emergency brakes, and start the engine. Let it idle for two minutes and then shut it off.
2. Pump the brake pedal at normal foot pressure four times and hold your foot on the pedal pressing down slightly on it.
3. Start the engine. As you start the engine, you should feel the brake pedal moving downward slightly, about an inch or less. Otherwise, you don't have enough vacuum in the brake booster. To locate the fault, do the Engine Vacuum and Brake Booster tests described in the following sections.
4. With the engine still idling, remove your foot from the brake pedal and turn off the engine.
5. Depress the brake pedal (using normal foot pressure) four times. If you notice the pedal rising after the second or third you depress it, the booster is more likely holding vacuum. Otherwise, a vacuum leak is affecting booster performance. To locate the problematic area, test engine vacuum and brake booster performance using a vacuum gauge and a handheld vacuum pump as described in the next sections.
6. Start the engine and let it idle.
7. Now, push down the brake pedal and turn off the engine, but hold the pedal depressed for about 30 seconds after shutting off the engine. The pedal should hold its position. If not, there's a leak in the brake booster, valve, vacuum hose, or intake manifold. Check the vacuum hose, booster check valve, engine vacuum, and brake booster as described in the following sections.

Mushy but does the pedal get more solid if you pump it once or twice? That's air in the system if that is the case.

 

[68 Sport Satellite]

Mushy but does the pedal get more solid if you pump it once or twice? That's air in the system if that is the case.

when the motor is running, no it does not get more solid when pumping the pedal. If I turn the motor off and pump the pedal it firms up after 3 presses of the pedal.

 

[68 Sport Satellite]

Had this happen to me a few times. Did all the checking and chasing. It ended up being a pin hole in the diaphragm. I can agree on the length of the pedal rod it must be the right length also. Back in the day rebuilt my own power booster's. Hope you get it figured out. I am running manual brakes and have plenty of stopping power. Disc. front and drums rear. Replaced the front with the wheel wood disc brakes. Love them.

glad you got it sorted. How were you able to confirm the pin hole in the diaphragm?

 

[68-500]

Are you rear drums adjust properly? If they don’t have a slight drag that could be your problem. I’d think if it was vaccum related the pedal would be hard not mushy.

 

[68 Sport Satellite]

Are you rear drums adjust properly? If they don’t have a slight drag that could be your problem. I’d think if it was vaccum related the pedal would be hard not mushy.

Thanks! I've come across this pointed out in a few articles and was told the rear shoes self adjust when applying the parking brake (which I always do). If not adjusted to slight drag, I wonder why it would cause a mushy feel? I know the front discs have a slight drag as I've checked this before, but not sure about the rears. A pro shop installed new rear cylinders and would think they would have addressed this, but need to check into it.

 

[dadsbee]

Rear brakes self adjust by backing up and stopping, not putting the parking brake on.

 

[68 Sport Satellite]

Rear brakes self adjust by backing up and stopping, not putting the parking brake on.

ok, good to know because the backup and stop and repeat was what I was taught long ago and I've done it each time I do brake maintenance. I wonder if the rear brakes can go out of adjustment or if there is fine tuning to get them to drag a bit via a manual adjustment?

 

[steve340]

I always adjust the drum brakes manually. If the drums are not adjusted up closely you find it hard to get the air out as the shoes "pump" in and out. Adjusting them may fix it.
Vacuum or the booster is unlikely to be the problem.
Vacuum gives you a softer to apply pedal not a spongy one if you get my meaning.
Failing that you could have a faulty master cylinder.

 

[TopBanana72]

Since you have wilwood disk setup on the front, could you contact wilwood about the booster and MC? Having all one vender might do the trick? So far your MC, booster and front disk setup and rear drums is a mishmash of vendors. They might recommend a better matchup. I think they have an adjustable proportioning valve too.

 

[68-500]

Also it’s my understanding if the drums are to far out of adjustment they won’t self adjust.

 

[68 Sport Satellite]

thanks to all of you - I'm going to manually adjust the rear drums and do a re-bleed, but not sure when I can get to it. When I do, I'll report back.

I'm also wondering if there's a clue to be had in the fact that when I added the additional vacuum can, the brakes got more whoosh whoosh sound and spongy feel starting from the top of pedal travel and also the pedal at rest was lower towards the floor. Why would increasing vacuum reserve capacity make things worse?

 

[steve340]

The only logical thing I can think of is the extra vacuum available to the booster made it operate better which highlighted the problem.
It is a bit hard to be sure as any input from posters/me is someone only trying to help based on your description.
Somewhat like trying to diagnose an engine knock in a telephone conversation.

 

[68 Sport Satellite]

thanks to all of you - I'm going to manually adjust the rear drums and do a re-bleed, but not sure when I can get to it. When I do, I'll report back.

I'm also wondering if there's a clue to be had in the fact that when I added the additional vacuum can, the brakes got more whoosh whoosh sound and spongy feel starting from the top of pedal travel and also the pedal at rest was lower towards the floor. Why would increasing vacuum reserve capacity make things worse?

The rear drum brakes adjustment checked out fine today. With rear tires in the air, E-brake off and trans in Neutral, engine not running, neither rear wheel spins freely. They both can be turned using both hands with enough drag from the brake shoes to only allow 1/8 - 1/4 turn at a time. Seems normal to me from videos I've watched.

So it's gotta be air in the system. 2 pro shops didn't get it right is tough to swallow, but then again, these Forged Dynalite Wilwood calipers with mounting adapters are aftermarket. I think I need to try the previous recommendation of bleeding the fronts with the bleeder port straight up in the air at a 90 degree angle from horizontal instead of at the mounted upward facing 75 degree angle, then re-mount when done. If I change fluid and do this re-bleeding at the new MC and all 4 corners and it's still a problem - I'm going to change the booster.