Hard Brake Pedal

[d.c.]

I am running wilwood brakes on an AME frame for a 57 chev. I have a hard brake pedal and can't seem to figure out why. Although the 502 ram jet is giving me the typical "green mode" problems i.e. rough idle extremely rich air fuel mix etc. I do seem to be getting a vacuum signal to the booster. My pedal ratio is admittedly off from the golden 6 to 1 benchmark, but using the vise grip and angle iron extension trick didn't seem to help. I can feel the pedal go down when I start it with my foot on the brake but after that it is rock hard. Any help would be wonderful.

[wicked68]

I am running wilwood brakes on an AME frame for a 57 chev. I have a hard brake pedal and can't seem to figure out why. Although the 502 ram jet is giving me the typical "green mode" problems i.e. rough idle extremely rich air fuel mix etc. I do seem to be getting a vacuum signal to the booster. My pedal ratio is admittedly off from the golden 6 to 1 benchmark, but using the vise grip and angle iron extension trick didn't seem to help. I can feel the pedal go down when I start it with my foot on the brake but after that it is rock hard. Any help would be wonderful.


you can possibly move the pedal rod higher up the brake pedal shaft for better leverage.

what brake calipers are you using and what bore mc do you have?

[d.c.]

Hi Wicked, No I cannot move the pushrod higher, and although I did give myself some extra leverage by clamping some angle iron to the pedal and pushing there, it did not seem to help. I do not know the bore size of the master cylinder which I suspect may be the issue but am hoping not. It is painted to match the frame and engine and we have a track day coming up in a week. Outside of the M/C bore size which I agree may be the culprit what else could cause this? I am using wilwoods six piston "big brake" calipers up front and the 4 pistion rears provided by AME.

[wicked68]

Hi Wicked, No I cannot move the pushrod higher, and although I did give myself some extra leverage by clamping some angle iron to the pedal and pushing there, it did not seem to help. I do not know the bore size of the master cylinder which I suspect may be the issue but am hoping not. It is painted to match the frame and engine and we have a track day coming up in a week. Outside of the M/C bore size which I agree may be the culprit what else could cause this? I am using wilwoods six piston "big brake" calipers up front and the 4 pistion rears provided by AME.


If you have mounted the mc at an angle on the firewall where the piston would be pushing at a slight angle to it would cause it to be hard to push as well and likely eventually wear out the bushing and cause it to leak.

if you have a 2 or a 1 7/8 its probably not bore size as the issue.

[silver69camaro]

DC, I believe most Tri-5's have a 8:1 pedal ratio for manual setups, which should be more than enough...power brake ratios can be much lower. What master cylinder size are you using? Exactly what pedal ratio do you have?

Also, check your vacuum to be sure about that as well. You can get a quick vacuum check by a quick engine rev, which will spike the vacuum...if the pedal feels good after that, then your engine doesn't make enough at idle.

To me, it sounds like a vacuum problem, or a booster issue.

[d.c.]

Hi Matt,
My pedal ratio is low, in the 4.5 to 1 range. Sadly giving myself an extension off the brake pedal did not seem to help. I have booked some dyno time this week so we can check for a vacuum signal then, but I suspect it is a m/c issue myself and both you and wicked have confirmed that. BTW the frame you guys supplied was one of the few things on this build that went smoothly and everybody gives me the "nice" comment when they see the chassis setup. The few tech questions I had were always answered quickly and properly. I will be sure to you AME in the future.

[silver69camaro]

Glad everything went well. Hard to believe we've sold almost 750 of those things!

Let me know how things go or if you need further help.

[1BAGTO]

I have the issue in my GTO that silver69mentioned above. Once I am at speed or "reving" the pedal is fine, but pulling out of the driveway is a bitch.

[d.c.]

So did some dyno testing on the 502 Ram Jet today and came out at 373hp and 401ft/pds. Not to bad for rear wheel horsepower at an elevation of about 3500ft. At any rate the hard brake pedal is still an issue that I haven't solved yet. We are pulling 10 inches of vacuum at idle which is obviously low, so here is what I am thinking. Please correct me if anyone has a better solution. I only have room for a 7 inch booster so I considering first and foremost a dual diaphragm version in the same size. I have discounted a vacuum can because we only have 10 inches of signal at idle and it would take a lot of deccelerations to gain a strong enough signal to operate the current booster. I am also shying away from a vacuum pump because of both the cost and that I have been told they are noisier then a room full of five year old girls.

So, does my logic sound correct? Any input would be great. BTW I am running Wilwoods 6 piston front and four piston rear caliper setup.

[wicked68]

get rid of it and go manual :D

[MarkM66]

get rid of it and go manual :D

No doubt!!

[silver69camaro]

Yep, sounds like a manual setup is the way to go. The problem with small diameter boosters is they need more vacuum to work properly, compared to a larger one. 10" is way too low.

[d.c.]

10 inches is low. I am suspect that I may have a vacuum leak somewhere. The engine is a 502 Ram Jet and the cam isn't all that big in it. Anyone have an idea as to how much vacuum these pull?

[Skip Fix]

My TA's factory small dual diaphragm works perfect with 9" vacuum, 8" starts getting a little iffy at times. They actually used less pedal ratio for them over the single diaphragm boosters.