Brakes - Head scratching

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Hi everyone. Your thoughts please. (BTW not many Help threads. Only one this year)

In any case:
- on the coupe I've been working on forever, I have an underfloor master with dual diaphragm booster.
- disks on all four corners. GM 154 style calipers in front and Wilwood 4-piston rear
- residual check valves front / rear
- 14" Vac at idle (383) - tested at the vac hose at booster location.

Issue
- after bleeding numerous times and without engine running, pedal is very firm
- with engine running, pedal is very soft and doesn't get firm until at least 50% pedal travel.

I have to assume that the booster is working as the very hard pedal disappears after start up BUT why the excessive travel?

Thoughts?
 
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Frank. You also need a check valve to the front because your m cyl is lower than the calipers. Also….push rod length?
 
Keith, I have a check valve to front. Sorry, forgot to specify - made edit

Push rod length as in between master and booster?
 
Yes. Push rod between master and booster. As I recall residual check valves for disc brakes are 2 lbs.
 
PG409 said:
Yes. Push rod between master and booster. As I recall residual check valves for disc brakes are 2 lbs.
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Push rod between MC and booster was done correctly on install. Res check vales are 2 lbs. After talking to a bunch of people, most insist this condition is indicative of air still in the system somewhere.
 
Frank, I had a hell of a time bleeding my (4) 4 piston wilwood calipers on my project. I bench bled the MC about 4 times and still couldn't get a hard pedal. I was assuming that you only needed to bleed through one of the 2 bleed screws on each caliper. Finally I bled all 8 bleeders and finally got a somewhat hard pedal. It seems soft to me but I am hoping with the hydroboost assist it will feel better.
 
ratrig said:
Frank, I had a hell of a time bleeding my (4) 4 piston wilwood calipers on my project. I bench bled the MC about 4 times and still couldn't get a hard pedal. I was assuming that you only needed to bleed through one of the 2 bleed screws on each caliper. Finally I bled all 8 bleeders and finally got a somewhat hard pedal. It seems soft to me but I am hoping with the hydroboost assist it will feel better.
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Lee, I understood that you have to bleed only the top two on the 4-piston calipers - outside first then inside is what they recommend.

What I don't get, hence the head scratching, is why I have a very firm pedal with the car not running then soft after starting. If the booster is pooched (it's brand new), you should still have a hard pedal. The booster is clearly operational but lots of travel before feeling some resistance.

As I said above, lots of opinions that there is air compressing somewhere. I've done three THOROUGH bleeds.
 
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Frank, This reminds me of when my brother took in his new Kia Stinger when his brake pedal did the same ,hard when shut off and mushy when he started it. The service guy and bro went out to the lot and tested other new cars and they did the same but the brakes worked fine. ????
I just went out and pressed on the brake pedal on my 2014 Ram with Vacuum booster and it was hard, then dropped when I started it. But the brakes work fine. Have you tried driving it? Or is it still in a build stage?
 
ratrig said:
Frank, This reminds me of when my brother took in his new Kia Stinger when his brake pedal did the same ,hard when shut off and mushy when he started it. The service guy and bro went out to the lot and tested other new cars and they did the same but the brakes worked fine. ????
I just went out and pressed on the brake pedal on my 2014 Ram with Vacuum booster and it was hard, then dropped when I started it. But the brakes work fine. Have you tried driving it? Or is it still in a build stage?
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Lee, the pedal will always drop when you start a boosted car BUT my pedal is soft through at least half it's travel then you feel firmness.

@shane2290 that may be an option - the power bleeder.
 
Pushrod location on pedal arm?
Could you move the pushrod mount on the arm closer to the pedal?
Standard brakes need more leverage than power assisted.
 
I hadn't considered that Sassy but yes It's quite likely that the mechanical advantage needed for non power would be different .
 
Sassy said:
Pushrod location on pedal arm?
Could you move the pushrod mount on the arm closer to the pedal?
Standard brakes need more leverage than power assisted.
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Not following Sassy. This is a booster application. That’s the issue.

Engine off = firm pedal

Booster = lots of travel
 
On Comets and Falcons that I'm familiar with the pushrod is mounted closer to the pivot point for more pressure on standard brakes that use a smaller diameter master cylinder.
The pushrod is mounted closer to the pedal in cars with power brakes to increase the distance the pushrod travels on a larger master cylinder.
Master cylinder diameter plays a big roll in pedal travel and all wheel discs need more Dia than disc/drum.
I think all cars with a vaccum booster have a hard pedal with the engine off.
 
Sassy said:
On Comets and Falcons that I'm familiar with the pushrod is mounted closer to the pivot point for more pressure on standard brakes that use a smaller diameter master cylinder.
The pushrod is mounted closer to the pedal in cars with power brakes to increase the distance the pushrod travels on a larger master cylinder.
Master cylinder diameter plays a big roll in pedal travel and all wheel discs need more Dia than disc/drum.
I think all cars with a vaccum booster have a hard pedal with the engine off.
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I had a vacuum booster on my '52 due to low manifold vaccum at idle, pedal was soft with the engine off.
 
Do you have a 2psi Wilwood valve in the front? If so check and see if it's working. I have had a ton of them fail.

One thing you might try, clamp the front wheel calipers so the pistons are retracted as far as they will go, and then bleed. I have used this trick to bleed difficult systems
 
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