Spongy Brakes
 

Can some-one help with this, my pedal has always been very spongy since i moved to a 7/8 master.. my brakes are manual , pedal ratio changed ,, just installed willwood 4 pot 'dynalite discs' and calipers up front and 'right stuff' single piston rear calipers .. Master is wilwood , and using a summit proportioning valve. I did email wilwood and they have suggested that the 7/8 master may not be enuf with the larger rear piston calipers..

could this be correct?? i need more pedal..


Gav

Was the pedal OK before the new master or did you install it when you installed the Wilwoods and Right Stuff discs? Were you ever able to get the parking brake adjusted so you had 1/2" or less travel of the park brake actuating levers on the calipers?

There are folks here who know far more than I ever will about brakes who might have answers for you but more info is required. Many knowledgeable people don't want to start a week long fishing expedition asking questions to get the info they need to help you. Take some time to list in detail everything you've installed and the methods you used to install & bleed include pics of everything. Then you may get the answer you need.

Was the master cylinder bench bled according to Wilwoods instructions before installation?

What method was used to bleed the system on the car?

Spongy usually equals air trapped in a high spot or pocket in somewhere in the system. Sometimes it's a real challenge. Altering the direction of fluid flow and push vs pull may remove a stubborn air pocket.

As Scott stated, spongy generally equals air. That said, an undersized MC relative to the caliper piston area can make the pedal feel "bouncy" for lack of a better word, which could be confused for "spongy". We can go back and forth over semantics, but that's probably not productive :catfight:

If you know your piston diameters or areas, please share, otherwise we're forced to make assumptions. My best guess is that your Wilwood Dynalites have 1.75" pistons and therefore 4.80 square inches of effective piston area. As for the rear calipers, are the RSD kits using the 78-85 Caddy calipers or something else? Just based on your front calipers, the 7/8" bore MC is on the small side of things...if the rear calipers have 3.00 square inches or more area, then you're definitely undersized on your MC. That's not to say that it won't work, it just may not provide the feel that you desire.

Tobin
KORE3

Tobin, this is the caliper RSB is sending. I'm building a 69 Camaro for a customer who had purchased these rear brakes and wanted them installed on the 12 bolt I built for the car.

http://i240.photobucket.com/albums/f...MVC-032F-1.jpg

http://i240.photobucket.com/albums/f...MVC-027F-1.jpg

Guys , im not sure on rear caliper make ,,right stuff kit with the adjustment arm for emergency brake, largish single piston,, i have bench bled master 3 times now out of frustration,, vacuum bleed, reverse bleed and good ol' 2 person bleed has been done prob 10 times with no luck.. found a couple of bad lines and replaced them,, changed pedal ratio.. I was running the wilwood 7/8 master with rear drums and stock camaro discs before i changed to this current setup and was never happy with the pedal.. it was always spongy and too close to the floor . The emergency brake is not fully adjusted ,, prob running 50% of what it could be.. thats another problem, getting these things to 'wind up' is an art in itself ive found. Would that solve my problem?
P.S. this is a manual setup as my injection runs very little vacuum for assist.
I have also tilted the rear caliper nipples upward when bleeding as per instructions just to squeeze that last bit of air from system.. every trick i can think of..

Short and simple. The master cylinder must displace as much (preferably more) fluid than the amount of fluid it takes to push the pistons out enough to apply good pressure to the pad / rotor combo.
My bet.... The m/c is too small.

This is the conumdrum.... :lostmarbles: some are saying it is,, others are telling me 7/8 is what i should be running? is there a difinitive answer?

I wouldn't do anything with respect to the master cylinder until you have the rear calipers set up properly, since that's more than likely exacerbating any perceived volumetric issues that you may or may not be experiencing. The rear pads should be virtually snug up against the rotors, but not tight. See link below.

http://www.pro-touring.com/cars-gene...rs-101288.html

While there are definitive answers with respect to MC sizing, your system should work, albeit with a somewhat longer pedal travel than some would like. If I were making a recommendation for your system from scratch, I would suggest a 15/16" bore unit for manual brakes with a 6:1 pedal ratio. That said, there is only a ~12% difference between that and a 7/8" or 1" bore unit and it sounds like your pedal travel issue is greater than 12%, making me believe you primarily have a rear caliper installation issue.

Tobin
KORE3

Here's a simple question. Anybody here running the 7/8" Wilwood master cylinder with a manual brake setup that doesn't feel a little spongy? I have that same setup and mine is slightly spongy also. I was doing some research on it a few months ago and it seems that it's a fairly common side effect as you reduce the bore size on a master cylinder in a manual setup(regardless of the calipers). As you increase the bore size, you get a harder pedal, but less brake pressure.

I'm sure there are plenty of people out there are running this master cylinder and bore size OR have done so in the past that could chime in. Would love to hear from someone who has this exact setup(Wilwood MC, 7/8", manual brakes, 4 disc) and can say they have a hard pedal.