What should Brake Pressures be? - Vintage Mustang Forums
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post #1 of 31 (permalink) Old 04-16-2018, 04:08 PM Thread Starter
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What should Brake Pressures be?

Sorry so long, but there is a lot of details...I have a SSBC power front disc / rear drum conversion kit on my 1966 Mustang 289. I have 18" Hg vacuum for the booster (stock cam), a 10 psi residual valve and a proportioning valve (PV) on the rear line, have bled (many times) and adjusted the rear brake shoes to be just dragging and adjusted the booster rod gap using the Baer H tool.

The problem is brake pedal free play at the top of the pedal where it goes down about 2" before any pressure starts to develop from the master cylinder. Today I ran a test where I installed a SSBC brake pressure gauge in the left front caliper and the left rear cylinder, started the engine and recorded the pressures using the booster.
Test were as follows:
1. Pumping the pedal in and out for 1" of pedal travel: there was no movement of the pressure gauge front or rear.
2. Pumping the pedal in and out for 2" of travel: I saw a blip on the pressure gauge of 50 psi, so I now know the MC starts to output pressure at 2" of pedal travel.
3. Without adjusting the rear PV (that was set at 1-1/2 turns out normally), I placed my foot on the pedal and pushed in until I met solid resistance at about 3" pedal travel and reading 50 psi, then I continued pushing as hard as I could. The front read 900 psi and the rear read 700 psi. Then I did an emergency stomp and read 1200 psi front and 700 psi rear.
4. I repeated test 3 above with the PV unscrewed all the way out for maximum decreased rear pressure. The front read 900 psi and the rear read 500 psi. Then I did emergency stomp and read 1200 psi front and 500 psi rear.
5. I repeated test 3 above with the PV screwed all the way in for a maximum increase in rear pressure. The front read 900 psi and the rear read 750 psi. Then I did an emergency stomp and read 1200 psi front and 750 psi rear.

Is this normal? I now understand why I have 2" of pedal free play at the top because there is no pressure getting to the cylinders until the pedal moves 2", however, I think it should start sooner. (Note: The pressure gauge doesn't fit the MC so I can't measure there to compare MC pressure to wheel cylinder pressure.) I was surprised I could only develop 750 psi in the rear cylinders even with the PV set at the maximum increase in pressure. Wonder if the PV is bad? Good news is the rear shouldn't lock up.

2 Questions: Is the only adjustment governing the top pedal free play for when you develop pressure from the MC the booster rod/MC gap and is this controlled by porting inside the MC and the piston? And what is the max pressures I should get from the MC for the front and rear brakes?
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post #2 of 31 (permalink) Old 04-17-2018, 07:16 PM Thread Starter
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Not getting any responses, so I'll boil it down to one question:
Does anyone know if 900 psi on front calipers and 750 psi on rear cylinders is within acceptable operating limits?
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post #3 of 31 (permalink) Old 04-17-2018, 07:59 PM
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I believe disc requires 900-1200. This is one link:

Wilwood Disc Brakes - Frequently Asked Questions (FAQs)

Has the following:
Disc brakes require approximately 900-1200 PSI at the caliper for effective functioning.

Does your have and adjustable brake pedal push rod? Maybe that needs some tweaking.
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post #4 of 31 (permalink) Old 04-17-2018, 08:27 PM
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Originally Posted by poqman View Post
Not getting any responses, so I'll boil it down to one question:
Does anyone know if 900 psi on front calipers and 750 psi on rear cylinders is within acceptable operating limits?
Rear sounds about right, the front I got around 1200 when pressing really really hard. I think your pressures are ok.

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post #5 of 31 (permalink) Old 04-17-2018, 08:30 PM
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Check out this thread:

http://forums.vintage-mustang.com/vi...ng-issues.html

65 coupe, M-6007-X302 340 hp crate engine
Street Or Track front Bilstein coilover system

https://www.youtube.com/watch?time_c...&v=7HztIgJymNw
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post #6 of 31 (permalink) Old 04-17-2018, 10:22 PM
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I just remembered, I have a 15/16 master cylinder so mine would develop a bit more brake pressure psi than what I assume is your 1 inch master cylinder.
Oh, and my measurements were with a stock brake pedal.
Have you called SSBC?

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post #7 of 31 (permalink) Old 04-18-2018, 07:41 AM Thread Starter
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Quote:
Originally Posted by green1965 View Post
I just remembered, I have a 15/16 master cylinder so mine would develop a bit more brake pressure psi than what I assume is your 1 inch master cylinder.
Oh, and my measurements were with a stock brake pedal.
Have you called SSBC?
I do have the 1" MC. I did call SSBC and they were not helpful. They kept telling me their company was founded on this kit and how great it was. Obviously not my experience and no offer to exchange any parts.
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post #8 of 31 (permalink) Old 04-18-2018, 07:44 AM
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Originally Posted by green1965 View Post
I just remembered, I have a 15/16 master cylinder so mine would develop a bit more brake pressure psi than what I assume is your 1 inch master cylinder.
That is what I was going to mention - MC bore plays a big role in pressures. Sounds like your pressures are good though.

Maybe a longer push rod to reduce the pedal travel?
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post #9 of 31 (permalink) Old 04-18-2018, 08:00 AM Thread Starter
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Does your have and adjustable brake pedal push rod? Maybe that needs some tweaking.
I do have the adjustable brake pedal rod and adjusted rod length so the pedal height is 2" above the gas pedal and gas pedal is set at 3-7/8" above floor (as set by Ford service manual). I can make the pedal higher, but it then doesn't feel natural when sliding foot from gas to brake. At this height, the pedal is not against the bumper stop.

I did set the MC to booster rod gap at .005". That is all the gap I could get. SSBC wasn't happy about that and wanted .015" gap. To my thinking, 0 to .005" gap should be fine and Ford service manual agrees with me.
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post #10 of 31 (permalink) Old 04-18-2018, 08:04 AM Thread Starter
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Thanks for inputs. Since pressures seem to be within tolerances, I will order higher friction pads and shoes to help get better stopping power. I hear EBC RedStuff pads and Porterfield R4-S shoes work well on this car.
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post #11 of 31 (permalink) Old 04-18-2018, 08:55 AM
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I do have the 1" MC. I did call SSBC and they were not helpful. They kept telling me their company was founded on this kit and how great it was. Obviously not my experience and no offer to exchange any parts.
No surprise there. I had a similar experience with SSBC and will never buy their products again. I don't understand how they are still in business.
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post #12 of 31 (permalink) Old 04-18-2018, 09:25 AM
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1200 psi should be enough to lock all 4 wheels up. Are your brake lines rubber stock & new? If they're stock or old I'd replace them with some new braided lines. They can look good to the eye & be bad with 900-1200 psi they can bulge with that much pressure causing to much travel on the brake pedal (just my opinion from having it happen) I put new brake 4 or 5 layer braided lines on front and back last summer made a huge difference in how the pedal felt.

I pm'd Zray yesterday about pads his opinion is the Porterfield 4r-s were better than the red stuff EBC but not much for the price difference.

I have EBC slotted & drilled rotors with some kind of aftermarket black pads from the previous owner wearing a groove in the rotors. I found my ball micrometer yesterday and have enough to turn the rotors once.

I ordered the red stuff EBC pads from AZ yesterday they're special ordered 24-48 hrs to get them for about $40 cheaper than anyone else online. I'll be getting the rotors turned in the next couple weeks.

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post #13 of 31 (permalink) Old 04-18-2018, 09:46 AM
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Originally Posted by Bob & Sue View Post
"......I pm'd Zray yesterday about pads his opinion is the Porterfield 4r-s were better than the red stuff EBC but not much for the ......"

Bob
That's backwards from my PM . I like the ebc redstuff better, but the difference is not great, IMHO.

PM "...I like the ebc red stuff pads the most.......A close 2nd would be the Porterfield R4-s pads.
Summit has the ebc, and once you find out the right part number, Amazon might have the best prices...."

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post #14 of 31 (permalink) Old 04-18-2018, 09:58 AM
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That's backwards from my PM . I like the ebc redstuff better, but the difference is not great, IMHO.

PM "...I like the ebc red stuff pads the most.......A close 2nd would be the Porterfield R4-s pads.
Summit has the ebc, and once you find out the right part number, Amazon might have the best prices...."
Sorry about the mis-quote my bad. I owe you one.
Should of had some more coffee and read the pm again before typing.

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post #15 of 31 (permalink) Old 04-18-2018, 10:18 AM
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