Very hard brake pedal marginal stopping

[Dennis Mosley]

Hi guys,

I purchased a 13 year old cobra that had some pedal issues. I replaced the brake pedal with a Tilton unit, replaced the master cylinders with new .75 Wilwood units that have remote reservoirs. The front calipers are 4 piston Wilwood Superlites and the rear are Jag.

The problem is that once hooked up filled and bled, the pedal is super hard. Doesn't feel like it moves more than 1/2 an inch. Also if I'm gong greater than 10 miles an hr, I can't lock of the brakes.

There is no power booster so that can't be bad.

I've adjusted the push rod out to pretty much the very end. I've adjusted the balance bar so that it is bias to the front brakes to try to get better stopping.

Before the lines were attached, the pedal had free travel so I know there is nothing in the way of the pedal moving.

Any tips on what I should check and why the brake pedal is so hard?

Thanks

[Dominik]

I assume your pedal ratio is suitable for non-booster application, like - let me thumbsuck - 7:1 instead of 4:1?

Does somebody have the correct ratios?

[Dennis Mosley]

got the same advise form a local page. went and checked and it was around 5. The pedals are adjustable so I maxed it out and I'm around 6.1 now. I will try that out in the morning.
Thanks

[legenmetals]

You may want to check your brake line pressure and if you have enough than I would look at your brake pad compound and rotors.

[mln385]

Did you bench bleed the master cylinder prior to install?

[strictlypersonl]

I would check the free play again. The pushrods into the master cylinders must have play in the rest position or the pressure won't be released.

[joyridin']

Are you sure your calipers are good? Sounds like the rear calipers are froze.

[t walgamuth]

My brakes feel fine but won't lock up no matter how hard I try. I am planning to go to a size smaller on the master diameter.

[spdbrake]

From the Wilwood site troubleshooting matrix:
HAVE TO PUSH TOO HARD ON PEDAL
Too large of a master cylinder. See master cylinder web page to select the correct size unit for your application.
Not enough pedal ratio. Increase pedal ratio, see pedal technical tips.
Pedal mounted at bad angle. Master cylinder push rod should not be off more than 5° in any angle
Wrong pad material for your applications. Pads must match rotor operating temperature range. See brake pad web page.
Frozen pistons in calipers. Rebuild calipers.
Wilwood Disc Brakes - Tech Troubleshooting

Although it seems strange the 1st thing to look for on the T/S list "Too large a master cylinder". The OP did mention going from a single to twin 3/4" masters.

From the How to select page:
Wilwood Disc Brakes - Frequently Asked Questions (FAQs)
Q: What master cylinder bore size do I need?
A:

The master cylinder bore size that you need is dependent on several variables. Choosing a master cylinder bore size begins with defining how the master cylinder will be actuated, manually or with the assistance of a booster, either hydraulic or vacuum operated. Once you have decided on how the master cylinder will be actuated, the information below can be used as a guideline for selecting the right master cylinder. Keep in mind that auto manufacturers have put many years of experience and a lot of testing into determining the right combination for a given car. When building a custom car with changes to the suspension, brakes, tires, and weight balance; you too may need to do some testing to determine which master cylinder is right for your vehicle.

The combined piston area and piston volume of your calipers and the pedal ratio, are two of the primary considerations. Whether the system is boosted or not is another consideration. Remember that a larger master cylinder bore produces more volume and a smaller master cylinder bore produces more pressure. To determine the appropriate master cylinder bore for your specific application, review Wilwood's Tech Tip Guide. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.


Q: What master cylinder is best suited for my application?
A:

The goal is to select a master cylinder system that supplies sufficient fluid volume to provide a firm, responsive pedal, while generating enough pressure to stop the car comfortably. There are two types of master cylinders, single reservoir, single outlet master cylinders and dual reservoir, dual outlet (tandem) master cylinders. A tandem master cylinder will have two fluid output ports, one for the front brakes and one for the rear. A single fluid output master cylinder, or "fruitjar," will be plumbed to all four calipers as a single system.

Dual master cylinder pedal systems are used extensively in racecars and have completely separate master cylinder systems for the front and rear brakes. This setup permits easy adjustment of front to rear brake bias with the integrated balance bar assembly.

To determine the appropriate master cylinder and hydra-mechanical combination, review Wilwood's Tech Tip Guide. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.

[strictlypersonl]

What is the piston diameter in the front Superlites? Hopefully 1.75"...

[joyridin']

I have just about the same set-up and I have .75" front and .625" rear with the same calipers on front and rear.

If I push hard enough, they will lock up, but it takes a lot to get it to happen.

[strictlypersonl]

Originally Posted by Dennis Mosley

Hi guys,...
I've adjusted the push rod out to pretty much the very end. I've adjusted the balance bar so that it is bias to the front brakes to try to get better stopping.
Thanks

Please elaborate, with some kind of a diagram, preferably.

[pormgb]

If you have two .75" master cylinders, that's going to cause an issue. I have Jaguar IRS with stock calipers and Wilwood Dynalites (4x1.38" pistons) up front, my master cylinders are 3/4" on the rear and 7/8" on the front and my balance bar is set a little to the front.

It took me a long time to get my bias correct, if you are pushing your balance bar to the front, it means you don't have enough line pressure and you front cylinder is too large compared to the rear.

I think the rule of thumb is that you have between 1/4" and 1/2" cylinder size spread between front and rear because the front caliper piston area is always larger than the rear.

Piston Area Calculator | Total Brake Caliper Piston Area | BRAKE POWER

Larger master cylinder moves more fluid but has a lower line pressure, the smaller cylinder has higher pressure but moves less fluid. Because the front calipers do more work, they need higher clamping pressure.

You can either do some math or go trial and error, if you increase the size of your rear master cylinder, that should provide better balance.

[Dominik]

Defenitely check free play. The realease valve must be able to open.

Nice advise on the front versus rear piston diameter. I am just busy building a car with the 427 master cylinder setup.

Ratio 6:1 sounds right. The difference to what you had before ist not drastic enough to explain the errors. But, as always, it probably a combination of more than one input.

Post a pic?

[Bartruff1]

My builder added a vacuum boost to solve that problem....it did...

[Dennis Mosley]

Well here is a little update. Talked with the folks at Wilwood and they suggested that I continue to use the 3/4 master on the front and use a 7/8 for the rear. Also said to check the pads and rotors to see if they looked like they had been broken in properly. No sure what that would look like. but I changed out the rear MC and that had no effect. so I pull the front tires to check the brake pads and rotors. Both the pad and the rotor look almost brand new. One thing I noticed was that when I pulled the pads, the pistons didn't move. This is my first time working with Wilwood calipers but I have never pulled brake pads and the calipers all stayed compressed. Pads had an orange substance on the back that looked like liquid gasket material. Guessing it was some kind of anti squeel stuff.

Called a friend of mine over and with the brake pads removed and a makeshif spacer in place as to not totally blow out a piston, I hit the brake pedal and on the tested caliper, 1 of the 4 pistons moved. we cleaned up a lot of the orange stuff from the pistons and with me pumping the brake we were able to get the others to move a little.

I'm guessing the calipers are as old as the car and given the 1 of 4 pistons moving when the brake is applied, I'm pretty sure that would contribute to poor stopping.

I plan to chat with Wilwood on Monday but I'm guessing a rebuild is in order.

Thanks for all the above ideas.

[Gaz64]

Sounds like half the pistons are seized, from lack of use/lack of brake fluid flushing.

Gary

[kyleb]

Disconnect the line from the caliper and see if the pedal pushes fluid through the flexible line ok..
That way you know if the problem is before the caliper or not and also the telltale if then the pistons will push back into the caliper as well.
I just had to replace a flexible line to the caliper that had blocked internally due age on my Mustang.

[joyridin']

The rear brakes are where your pedal level comes from. If the front is frozen or partially frozen, the rear are going to be the same.

[Dennis Mosley]

KUDOS to Tilton Tech Support!!!!!

Not as happy with Wilwood Tech.

Called Wilwood tech first since I am using Willwood calipers and master cylinders. Asked me what kind of car and model of the calipers and sizes of the MC's I was using. His response was, that I needed to switch the back MC to a 7/8" and "rough up" my rotors and pads and that should work. BUT if it didn't work I should call tilton since I was using their pedals. I give it a try and it doesn't work so I call tilton as instructed.

I call and get Gary on the phone. He asks me a few questions and said, "are you telling me they just punted on this cause you have our pedals...hydraulics are hydraulics" He told me he would get me fixed up. He asked for the model numbers of the MC's I was using and he and I pulled them up on line so he could see the specs. Then he told me he wanted me to fill out a spec sheet for my car that he emailed me and it would help him. The sheet included thinks like # of pistons in the caliper, size of piston, rotor size, car weight and weight distribution and some other stuff. He also asked what kind of pedal feel I wanted. I had to look up some of the stuff but emailed him the sheet that evening and the next morning he provided me with his recommendations. Actually gave me 2 options depending on the type of pedal feel I wanted. The one I chose recommended going back to 3/4" on the back (going with a 13/16" on the back would give me a heavier pedal feel) and going to a 5/8" on the front. Didn't try to sell me anything.

Made all the changes last night, pedal feel is what I was looking for and when I tested the brakes, at 45 mph and a "Panic" stop, wheels lock up as I would expect.

All that info may or may not have been needed but all I can say is I feel like he put in the effort to fix my problem and that's the kind of service I like to get.

You can be sure that the "frankenbuild" project I have going on in the garage will have tilton components in the braking system.

Great job Gary and Tilton as a whole.