jkg2101

i have a 67 with manual brakes. i have a new master cylinder (exact repro). i have all new brake lines. there are no leaks. i have 4 new wilwood calipers, which are identical piston size to stock

i have not been able to get a firm pedal. the pedal travels far, with low pressure, but does not bottom out. when driving the car, the car will stop but a panic stop is not good - does not stop quickly, one of the rear tires might just barely lock/skid, but the fronts definitely do not skid at all

i replaced the master with another new one because i thought it might be seeping

i have bled the system a dozen times. with a mity-vac. with a pressure on the master cylinder device (forgot the name). also just used my son on the brake pedal
i unbolted the calipers and made sure they were vertical to get all air out.

i put a pressure gauge on the calipers. on the rear, i get up to 500 psi. on the front i get 300 at most. not sure how much pressure i should be seeing. this is with a firm push on the pedal.

the only original part of the system is the distribution block. i dont know if it is possible to have that thing be the problem

1. what is the correct psi i should see with manual brakes?
2. any idea why i cant get a good pedal or brake system?

it just feels like the master cylinder bore size is too small - but this is the correct master, so that is not the problem.

please help! justin

jim lockwood

300 psi on the front is kinda low. I'd expect to see the same pressure front and rear and 500psi or more is in the right ballpark. Look into why the front line pressure isn't any greater.

Your thinking about the master cylinder bore is reversed. A small master cylinder bore will give you more line pressure for a given pedal force than a large master cylinder bore.

For brakes which will send you through the windshield, you want large wheel cylinders and small master cylinder. But there are limits. Get the ratios of the wheel/master cylinder areas too great and you need more pedal travel than is physically possible.

Jim

jkg2101

I went back with my son and we both took turns really standing on the brake pedal. We both can get up to 700 psi on both front calipers But the pedal has a lot of travel. It does not bottom out but comes close. It just does not feel like a good break system when I drive it on the street I have a cobra with manual brakes of similar sizes and the breaking performance is so much better on the cobra

is 700 psi a good value to get? Can I have air in the lines and still get 700?

jkg2101

rongold

Do the wildwood calipers have 2 bleeders on the rear like the OEM calipers ??? If so, did you bleed both of them on both calipers ???


RON

jkg2101

dplotkin

Justin, here's what I'd do; replace the block to rule it out, which will require a re-bleed, which is what I think you need to do and keep doing until you get a good pedal. A good pedal is not always easy, especially on cars using non-stock parts, sometimes it takes repeated bleeding, working the brakes, bleed some more, to get it tight. Did you bench bleed the master? Has anyone fiddled with the pushrod adjustment screw, if any?

Dan

skids

skids

Factoid

jkg2101

i bench bled the master in the car several times

i did not open the bottom bleeder screws

does anyone know if 700 psi is a good number, and if it would be possible to see that number if there was air in the system?

buns

According to MBM Brakes, 700# is a good number with a 1 1/8" bore, but should be closer to 900# with a 1" bore.
.
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skids

In post six you said you bled them all; all 8 or 4?

ghostrider20

jkg2101

Thanks for the thoughts
i have built and bled dozens of brake systems over the years. i dont know what else to do, if i bleed it again. have done it over and over. i even bought another new master cylinder and started all over again. the only variable is the distribution block, since i just used the original, but i dont know any way that could be causing trouble?

solidaxel

I feel your problem!
We ended up changing Welwood pads to a softer compound and the third M/C ended up being a 15/16" down from a 1 1/8"
Never did a pressure test but was able to slide the front wheels after a few 100 miles
Good Luck !

Factoid

jkg2101

my understanding of this proportioning valve for the 67 is that it does not have a shuttle valve, but rather just a sensor that can detect brake imbalance. this is unlike later cars.

my question has been whether the gauge pressure reading of 500-700psi is consistent with air still in the system somewhere. i havent heard a conclusive answer

i guess my plan now is to disconnect the lines and remove the distribution block to either clean the heck out of it or consider throwing more money at the problem by buying a replacement.

dplotkin

It responds to differential pressure and can be full of junk or corrosion from years of sitting in a hydroscopic solution. Its the last remaining variable and I suggested in post 7 that you change it and keep bleeding. Focus less on pressure and more on the wisdom offered here by many folks who all agree...you have air in the system. It can be a bitch to get rid of it.

What are you running for pads, by the way?

Dan

Black_Magic

I have Wilwood calipers on my ’66. Remove the Power Booster and went manual using a Wilwood MC #260-13375-BL 15/16” Bore. No proportioning valve.
Did bleed with Motive Power Bleeder (all 8 bleeder screws)

Don’t know pressure but stops great.