Mushy Spongy Brake Pedal but no apparent vacuum leak

[Kern Dog]

My front calipers and rotors are
from a 1980's Ford Granada. Not
new. (got them for free). They hooked
up to the Mustang II R&P easily.
The spindles are 70's Pinto.
(super strong).
The 2lb check valves do not cause
the pads to excessively drag on the
rotors, but in some cases will give
the brake pedal a firmer feel. After
initially installing my brake system
(it's more or less custom and pieced
together), I did not have the check
valves installed. Tight footwell in
the cab required modification of the
pedal arm to move it closer to the
gas pedal, and lower it. This changed
the pedal ratio. After the mods the
pedal had a spongy feel, so I
installed the check valves. Pedal
is now firm, and the brakes work well.
View attachment 1422688
View attachment 1422689
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No offense….
Has that been out on the road yet?
Newer cars use expanders to hold the pads off the rotors slightly for less drag. You went the other way. To me, that is only going to cause problems.
One time I was trying different things. I adjusted my brake booster to eliminate all pedal slop. It felt great at home and driving in the back yard. On the road, the pads heated up and then the car almost stopped dead when I coasted. That free play that I eliminated allowed a small space so the pads didn’t press the rotors all the time.

 

[Kern Dog]

I have a firewall stiffening plate already installed for that purpose. The footprint on mine looks longer in the vertical direction than the one in the photo above. Would I be able to use mine or do I need a different one? I'm not sure I understand your original comment about using one for 4-wheel non-power drums.

There are different plates.
The dual diaphragm booster mounts up higher on the firewall so the center hole (for the pushrod) and the booster mounting holes are different.

Manual master cylinders and the A body booster mount lower down and use a different stiffening plate.

 

[68 Sport Satellite]

There are different plates.
The dual diaphragm booster mounts up higher on the firewall so the center hole (for the pushrod) and the booster mounting holes are different.

Manual master cylinders and the A body booster mount lower down and use a different stiffening plate.

Interesting! I didn't know that. If I re-bleed and still have issues, I'll try this route. Will order this plate from Dr. Diff and buy one of your spare single diaphragm boosters off you.

Wonder why the Dr Diff site indicates this for manual or only Hemi power brake cars. Mine is a 400 block low deck. Should still work I don't see why it wouldn't.

 

[Kern Dog]

When I get home, I can show pictures of the different styles.

 

[1STMP]

No offense….
Has that been out on the road yet?
Newer cars use expanders to hold the pads off the rotors slightly for less drag. You went the other way. To me, that is only going to cause problems.
One time I was trying different things. I adjusted my brake booster to eliminate all pedal slop. It felt great at home and driving in the back yard. On the road, the pads heated up and then the car almost stopped dead when I coasted. That free play that I eliminated allowed a small space so the pads didn’t press the rotors all the time.

Yes, with no hot fluid problems. The
pads won't heat up the rotors/fliud
until the pad pressure meets 5lbs or
greater.
The force applied when you tightened
up the booster pin clearance was
multiplied with the effects of the
booster. There's no telling how many
lbs of pressure we're being applied
to the pads, and that pressure would have been constant. (my guess is
more than 2lbs).
All a 2lb residual check valve does
is reduce the time it takes for required
pressures of fluid to reach the caliper
pistons, which results in better pad
to rotor reaction times, and a firmer
feel on the pedal.

 

[68 Sport Satellite]

Did some reading on stepped bore MC's. Seems like it's not a slam dunk. Some have tried this and also had a spongy pedal. Don't think it's the smoking gun to my problem.

Plans for now:
-Check and re-Teflon tape seal threads for front caliper braided Stainless lines. Confirm no swelling of lines.
-Re-grease front caliper slide pins
-Re-bleed brakes both bleeders on each caliper with front calipers off adapters and facing straight up at 12 o'clock.
(looking at possibly installing Russel speed bleeders in place of stock Wilwoods, but can't find one to fit the required 1/8-27 NPT thread size. My front calipers are Wilwood Dynalite 4-piston part # 120-6815)
-Possibly add 2psi residual pressure valve for front discs.

Also found this handy trouble-shooting guide on the Wilwood site:

CAUSE	SOLUTION
SPONGY PEDAL OR BOTTOMS OUT
Air in brake system	Re-bleed the system.
Calipers not bled with bleed screws straight up	Unbolt calipers and hold with bleed screws in the vertical position.
Wrong size master cylinder (too small)	Refer to the master cylinder web pages to select the correct size for your application.
Faulty master cylinder	Replace master cylinder.
Calipers not mounted square to the rotor	Re-align brackets parallel to rotor, or shim caliper.
Calipers mounted equal to, or higher than master cylinder	Install 2 pound in-line residual pressure valve.
Calipers flex excessively	Check pressure. Do not exceed 1,200 P.S.I.
Pedal ratio too great	Reduce pedal ratio.
Excessive spindle deflection in corners causing piston knock-back	Install 2 pound in-line residual pressure valve. Check spindles for warpage.

Did some more reading since weather is getting in the way of much progress.

Revised Plans (expanded from above):
-Check and re-Teflon tape seal threads for front caliper braided Stainless lines. Confirm no swelling of lines.
-Re-grease front caliper slide pins
-Install Russel speed bleeders. Have confirmed 1/4-28 thread will fit the 1/8"NPT on the Wilwood calipers. Will probably order a few extra pairs since it seems 1 out of 5 reviews stated they broke the fittings on install because sometimes it's tough to get them to seat all the way with the built-in thread sealant.
-Russel Speed bleeders for rear brakes. Need to confirm thread size on my rear drum 7/8" wheel cylinders. Anyone know if this is also 1/4-28?
-Re-bleed brakes both bleeders on each caliper with front calipers off adapters and facing straight up at 12 o'clock with 1" socket between pads during bleeding. Tap calipers, cylinders, and MC with rubber mallet to release any trapped air.
-Add 2psi residual pressure valve for front discs. Not typically needed for firewall mounted MC, but I've read more than one account where this got rid of some spongy pedal and gave better pedal feel.
-Add 10psi residual valve for rear drums (haven't been able to confirm yet if my Raybestos PG Plus disc/drum MC # has an internal 10psi residual valve already, but I've learned that these valves are no cumulative, so the only drawback to installing one for the rears is that would be one more junction where a leak could happen.)
-Earl's fittings to plumb in the residual valves. Seems that the typically supplied brass fittings are thin wall and can crack or break, so some are using Earl's Stainless fittings and having better luck.

Holding off on a booster swap for now until I try the above.

 

[1MYTGTX]

@68sportsatellite So did you fix your braking issue?

 

[68 Sport Satellite]

@68sportsatellite So did you fix your braking issue?

yes, I did and I'm so happy it's finally working correctly! Sorry I forgot to post here. I posted the update in a related thread. After my last update, I was under the car in my garage and I found a small leak at the bottom of my brass distribution block that I hadn't noticed before. The line connecting this to the rear drum brakes is the only line I hadn't replaced. I disconnected it and it had massive tension on it, so no wonder it leaked.

I took the car into a 3rd mechanic who had previously done some exhaust work for me and is really good at bending lines for trans coolers, brakes, etc. and asked him to run me new line. He did and connected the existing fairly new brass distribution block I had bought from MP Brakes some 6 years ago and it leaked! So I set about to find a new one.

All along, I couldn't get a good pedal, it was semi-spongy and never had good pedal feel or clamping force, but the car stopped ok. I chased and trouble-shot everything for years. Finally, I decided to get another new distribution block of the same type. Many of the offerings out there have poor reviews on the Summit Racing website with the same type of issue I was facing. Poorly machined brass. I finally found one with good reviews and installed. Problem solved! The first new one I had I guess didn't have the internal brass seats just right so it would not completely seal. Or maybe my stretched line when connected boogered the seat up and I had to start over?

Now I have good pedal feel, good stopping power, and no more sponginess. It was air in the line all along like many have told me and suspected. And no residual valves installed and no booster replacement. Thanks to everyone for your comments in this or other threads. It kept me motivated, re-directed my approach, and it finally worked out in the end.

Here's the one I had good luck with recently. What a relief!
Right Stuff Detailing PV71 Right Stuff Detailing Brake Proportioning Valves and Blocks | Summit Racing

 

[moparx]

i know i'm late to the party, but i believe the single diaphragm dart booster is considered to be 10" in diameter.