Master cylinder size

cobra 53 · #1 (**permalink**) 08-30-2018, 11:05 AM

Hello from Spain

Since a longer time I have a NAF 289 Cobra Replica. I have already made lots of repairs and works on it, also with changes of front and rear breaks, also rebuild the Tilton master cylinders.

My queries is:

The breaks are a bit soft and I do not have a good feeling about the breaking point, i.e. a bad feeling when I start to break.

The clutch needs a lot of pedal pressure and is very hard to push. It works regarding slipping and chattering but still it is hard to press the pedal.

Input:

Engine SBF 289 / 1966,

Gearbox T5 WC

Pedals from Tilton

Master cylinder FB, RB, CL are from Tilton

Clutch: Centerforce Diaphragm 10.5", Hydr. Throw out release bearing, , MC Tilton 3/4 series 74, Tilton Pedal 5.5:1 Ratio

Front breaks: Calipers Forged Superlite Wilwood 120.11134 Piston 1.75"/ 4.80, Rotors 12.19" / 0.81", MC Tilton 3/4 series 74, Tilton Pedal 5.5:1 Ratio (balance bar)

Rear breaks: GM Metric emergency break calipers Speedway 91031046, Piston 2.5"? 2.02"?, Rotors 11.75" / 0.81", MC Tilton 3/4 series 74, Tilton Pedal 5.5:1 Ratio (balance bar)

I am looking for a solution, probably the "MC measures" are wrong size?

I hope you can help, will be highly appreciated. Thank you very much in advance and best regards from sunny Spain

strictlypersonl · #2 (**permalink**) 08-30-2018, 11:32 AM

The brake components look like they should work fairly well. Your spongy pedal might be cured by looking here:
Brakes or Lack of...

Brake pedal pressure: This is a non-boosted system, and it won't feel (easy) like a modern car with boost.

cobra 53 · #3 (**permalink**) 08-31-2018, 04:26 AM

According to found mathematics formulas, the "theory" is as follows:

Pedal ratio 5:5:1

100 lbs pedal effort:
FB MC 3/4" 1062 PSI
RB MC 15/16" 677 PSI

85 lbs pedal effort:
FB MC 13/16 " 1078 PSI
RB MC 1" 700 PSI

Would these MC size in a car with 2650 lbs inkl. passengers and full tank generate enough break power?

strictlypersonl · #4 (**permalink**) 08-31-2018, 07:06 AM

Back-of-the-envelope calcs...

1060 psi > ~700 ft-lb torque on each front wheel. That should yield about 0.8 or 0.9 G, assuming the rear effort is proportional. (I didn't do those calculations.)

cobra 53 · #5 (**permalink**) 09-01-2018, 04:39 AM

Math calculation on Post nr.3 are wrong. My mistake.

At Tilton Racing I found a calculation form and filled it in and submitted it.
I'm curious to see what comes back.

Dwight · #6 (**permalink**) 09-01-2018, 06:18 AM

Change the brake fluid.
Brake fluid absorbs water even in a closed system like our cars.

It would be best to use a evacuation system. I have a OTC Fluid Evacuation? Brake Bleeding Tool from Summit.

Flush a couple of large container of brake fluid through the system.

I like dot 4 synthetic.
I put dot 5 in a 64 Plymouth for a friend at his request. A couple of weeks latter he came back and ask that I replace it with dot 4 because the brakes were spongy.
Change the fluid and the brake were good.

Air and water; the enemy of good brakes.

Eight years ago Mike brought his Cobra to me for general maintenance. I could not stop the car within a save distance. Bad brakes!
I changed the fluid and then I could lock up the tires. I was amazed at the difference.

my427cobra · #7 (**permalink**) 09-02-2018, 06:18 PM

Hi all. I recently submitted my Cobra data to Tilton:

Their response was:

Hello Greg,

With the information you have provided on the MCBS form, predictions for your master cylinder bore size are :

At 100 lb pedal effort: Front 11/16 “ MC and 1” rear MC with a 5.1 pedal ratio

At 85 lb pedal effort: Front 5/8” MC and 15/16” rear MC with a 5.1 pedal ratio.

This is a prediction and the final bore size will be determined by the driver testing to achieve the best braking combination. Balance bar adjustment will change front to rear brake characteristics, the final decision should be based on track testing.

Cheers
Greg

cobra 53 · #8 (**permalink**) 09-03-2018, 09:14 AM

Hi Greg

thanks for sharing the Tilton info page regarding your car.

Best calculator for MC on my opinion is the site from Joesracing. Easy to anderstand.

I am still waiting my results fom Tilton.

Search in the "Knowledge Center" page.

www.joesracing.com

my427cobra · #9 (**permalink**) 09-03-2018, 10:22 AM

Hi cobra 53. It's a holiday weekend here so you won't hear from Tilton til Tuesday or Wednesday.

Cheers
Greg

my427cobra · #10 (**permalink**) 09-03-2018, 10:30 AM

Hi Dwight. DOT 5 brake fluid is silicone based and not at all suitable for street use. Racing teams using DOT 5 fluid drain and flush their braking systems after every race.

Now DOT 5.1 is a glycol (synthetic) based fluid and is DOT 3 and DOT 4 compatible and is an upgrade to a street brake system.

Lots of data on the web about the performance characteristics of DOT 5.1 and other similar performance fluids.

Cheers
Greg