Expert in Brake Hydraulics Needed

[GreenSnakeDoc]

Two years ago I replaced the stock Mustang !! front brakes on my LAX (now B&B) with a kit that used 11" Granada Rotors and 1980's Camaro calipers and have had a mushy pedal eversince. I can do 3 or 4 pumps and the brake pedal will give a little more resisance higher up, and this is usually the big clue that there's an air bubble in the system. After pumping and vacuum sucking about 5 or 6 quarts of Dot 3 through the system, I have no improvement. So, I made some devices to clamp down on the rubber brake hoses to seal off the individual calipers. (I can describe these in detail later if anyone's interested.) To make a long story short, with all 4 calipers sealed off, I have a very hard, high brake pedal. Pushing very hard for 10-15 seconds does not result in any leaks. Conclusion, the Master Cylinder and brake lines up to the hoses are good and tight. Sequentially releasing the clamps on individual calipers and pushing on the brake pedal allows the brake pedal to depress further and become more mushier each time. With all calipers functional, the brake pedal "catches" about 3" from the floor and can be depressed almost to the floor.

So, does anyone think my Master Cylinder is too small? It's a new one, has a 1.0" piston and it's for a 78 Mustang II. The rear calipers have the stock 2.125" pistons, while the front calipers went from 2.625" pistons to 2.938" pistons. Does any brake expert think this increase in caliper piston diameter exceeded the capacity of thge Master Cylinder?

[CobraDan]

http://www.aa1car.com/library/2004/ic30444.htm
You might find sone answers here.

[Doug I]

What length of rubber hose is in the system, and how old is it?

The rubber could be expanding and that'll give you the mushy pedal feeling.

[xlr8or]

Exactly what I was thinking Doug. Sounds like old rubber flex line.

[jwd]

Originally Posted by GreenSnakeDoc

So, does anyone think my Master Cylinder is too small?

I'm definately not an expert but that is indeed your problem. I had the opposite problem. My brake pedal had very little movement, was hard as a rock and it took very heavy peddle pressure to stop the car. No way could I lock the tires up. I had a 1" M/C for the front and same for the rear (Wilwood set-up). I changed to 3/4" front and 5/8" rear. Now it's almost like having power brakes.

Jim

[lineslinger]

Originally Posted by GreenSnakeDoc

T
So, does anyone think my Master Cylinder is too small? It's a new one, has a 1.0" piston and it's for a 78 Mustang II. The rear calipers have the stock 2.125" pistons, while the front calipers went from 2.625" pistons to 2.938" pistons. Does any brake expert think this increase in caliper piston diameter exceeded the capacity of thge Master Cylinder?

Yes.

The required fluid volume offered by your MC, and required by your brake system, is inadequate.

The reason you have a tight brake pedal until you remove the line clamps is the fluid volume required is not being exceeded.

The master cylinder capacity is being maxed out out when you depress the pedal while your calipers and lines are requiring more fluid to develop the required pressure as designed.

A larger MC should solve your low brake pressure problem.

[strictlypersonl]

I assume that you're using only a single master cylinder setup?

If so, you may be right about having a too-small master cylinder. (I'm just comparing your setup with the ERA, which has a dual-master setup, factoring in using equivalent piston area.)

Our total master cylinder piston area is about 15% greater than yours.

Your caliper piston area is at least 50% larger than ours.

Therefore, your mechanical advantage (and thus the required pedal travel) is 75%+ more than ours. That's a lot.

I would also check that the front and rear calipers are square to the rotors.

[Michael C Henry]

That is pretty much what I have for front brakes. I have the Jag inboard rear brakes. I'm using two Tilton 3/4" master cylinders with an adjustable balance bar from the pedal and between the two master cylinders. When routing the brake lines take great care in not creating places where air can collect in a high point in the middle of the system. Normal operation of the brakes you are moving a small amount of fluid in a column of fluid with each stroke. The master cylinder pushes fluid column to the brake caliper cylinders. The piston will move towards the rotor. The packing will slide on the caliper piston to a point and then twist with the inner surface of the packing cling to the piston and the outer edge still confined in the caliper bore grove causeing a twist in the packing. When released, the packing around each piston in the calipers will try to twist back to its neutral point, that is the release of the brake shoes. A verry small amount of fluid moving back and forth. What happens when not in use air that in system will collect at the high point. When you go to bleed the system you are trying to flush fluid containing that air bubble. All the while, that air bubble is trying to return to that high point. The farther that high point or multiple points are from the end ( the bleeder point ) the more fluid will have to be flushed in an effort to carry that bubble out of the system. My brake master cylinder resivoir is mounted on the firewall the highest point in the system. My master cylinders are below the floor boards. The high pressure line then falls a small amount to follow the frame member forward then splits and goes to each front caliper with the caliper being the highest point at the front end of the brake system. Generally most air in the system works it way out of the system through the master cylinder and escapes in the resivoir letting gravity work for you. Bleeding is only necessary to remove air in system at the higher points up front near each caliper. Remember the plumbers law "**** flows downward". If you have a higher collection point in your system it may take a power bleeder to push out enough fluid out to get that air bubble out of your system. Then (for the most part) your done . No more air will get into fluid system unless you have leak or you interupt your system as in a major repair.
You can replace the brake pads by just pushing the caliper piston inward (one caliper at a time) and not seperating the brake line. I pry against the old pads being replaced pushing the piston inward. then I unbolt the caliper. Tie the caliper up , don't let it hang by the hose. After each caliper is loaded with the new padsand bolted back in place, operate the pedal to move the fluid refilling the caliper bring the pads into contact whith the rotor. If you haven't been refilling the resivoir the fluid returned to the resivoir should not overfill and should be at the correct level. If you push both calipers back at the same time chances are that that amount of fluid will over fill the resivoir and cause a spill.

[DAVID GAGNARD]

Yes.

The required fluid volume offered by your MC, and required by your brake system, is inadequate.

The reason you have a tight brake pedal until you remove the line clamps is the fluid volume required is not being exceeded.

The master cylinder capacity is being maxed out out when you depress the pedal while your calipers and lines are requiring more fluid to develop the required pressure as designed.

A larger MC should solve your low brake pressure problem.

Zacctly!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Had a similar problem when converting my 65 Mustang from drum brakes to 4 wheel disc brakes.........easy fix...........

Go to your local parts store and get a master cylinder for a 1980-1984 Ford F-250, make sure it is the cast iron type and not the newer plastic type.... This master cylinder has the big bore to push a larger volume of fluid as well as big resevoirs to hold enough fluid...
Have since used this master cylinder on 5 or 6 cars, works great.....

David

[SF_SN888KE]

Great thread. I have the opposite problem too, HEAVY brake pressure to SLOW down car. CAN NOT lock up brakes. Has Howe master cylinders - seperate f/r , looks like Howe 14's 7/8" bore, with Wilwood Dynalite front brakes and 1998-2003(?) Cobra Mustang rear disc brakes. Are my master cylinders too big? What size cylinder is the Mustang Cobra rear discs?

[Clois Harlan]

If you are running a 7/8" for your rears it maybe to big. I once had a single MC set up using a 1 1/8" Mustang Cobra MC and my brakes workded great. I now run two Wilwood MC 7/8" front and 3/4" rear and they work great. I also have a proportioning valve that I can dial up some extra rear brakes when I want too!

Clois

[Jerry Clayton]

another reason for low pedal is rotor run out---they knock the pistons back away from the discs and it then takes excessive pedal travel(master cylinder stroke) to just take up the slack!!!! with a single master cyl when the one end is pressurized you can't then push the pedal further to completely pressurize the other end.

Also the usage of a residual valve could help the situation---some OEM masters have these behind the fittings and some RACE types don't--

[Dangerous Doug]

I recently upgraded my brakes from GM (Camaro) brakes to SSBC A185S's, and the pedal is nice and firm. The Camaro brakes always felt a little mushy, and I thought I had air in the lines (even after pushing a ton of fluid through on bleeding). I had a mechanic check it out and he verifiied that the mushy brake sensation was common to the Camaro calipers, and that I had no air in my lines. The Camaro calipers have a single, large piston, wherein the SSBC's have dual smaller pistons. The net result is I push less fluid through to clamp them down.

This thread is right on. The spongy feel is coming from either too small of a master cylinder or too large of caliper pistons. I think a master cylinder replacement would be cheaper than replacing your calipers.

DD

[Michael C Henry]

So say the stroke of the brake pedal stays the same. You can always do some compinsation with the amount of foot presure used to aganst the pedal to affect the results. All air is removed from system and all lines are pressure resistant so they don't just buldge out ( that would feel spongy)when you apply the brakes. At one extreme a smaller master cylinder ( we're talking about piston diamiter) will develop more pressure but less fluid will be moved, requiring less foot pressure. You may not reach resistance (Brakes actualling contacting rotors and clamping ) before running out of pedal travel. A larger Diameter master cylinder will move more fluid but unless you apply more foot pessure will develop less line pressure. Too large a master cylinder and you come up aganst the resistance early in the brake pedal travel but line pressure will be lower and will require more pedal pressure. You may not be able to push hard enough foot pressure to clamp the brakes hard enoughnto lock em up. Some states say you have to be wearing shoes when driving , Guess why?

[SF_SN888KE]

Good explaination.
Ref my heavy brake pressure (push hard to stop the car!) ... I need heavy shoes while driving! Got some info from Wilwood tech, make sure you start with at least a 6:1 pedal ratio, check balance bar, air in system, etc. 7/8" MC should work should work on dynalite (4 pistons x 1.75"). Does not recommeded anything smaller than 3/4" MC (currently using Howe 7/8") pedal travel may increase. Will try 3/4" on the front first and see how it feels. Howe Racing recommends big calipers, big rotors, let them warm up ...

What brakes/Master Cylinder combo are owners with 15" wheels running?

[Bobcat]

Ron ... Ive got the setup from ERA as described above on my car just as it came from them ... and 15 inch wheels and have no problems with pedal pressure or stopping so far . However , I haven`t tried to bring it down from 100 + yet as I`m still breaking it in .... but don`t expect any problems.

Bob

[Michael C Henry]

I'm not a know it all But I have 16 ' PSE wheels and the 10 3/4" Granada rotors and mid sized sinzed piston GM calipers front and original inboard Jag rear. I'm usung two 3/4" Tilton MC with adjustable balance beam . And of coarse they would want large rotors and calipers, wouldn't everyone? Larger diameter means more laverage but also takes larger wheels for clearence.

[CSXDreamer]

This is GreenSnakeDoc back on line. I had trouble getting my password two Fridays ago so I could get to this thread at home over the weekend. To make a long story short, I had to register with a new name to continue to participate. In the mean time, I've installed a 1 1/16" diameter master cylinder, which unfortuneately is totally mushy in that I can push the pedal all the way to the floor. Unlike the previous MC, this one will not "pump up" allowing a higher line pressure. I suspect I did not get all the air out of the MC when I bench bled it, then "bench bled" installed on the firewall. I would get a long stream of bubbles when the piston was depressed, so I tried clamping the clear vinyl hose with flat pliers so the piston would draw from the resevoir instead of taking the same fluid and bubbles back. First check after getting everything back together was a very, very soft pedal, right to the floor. So, I ran a quart of Dot 3 through the system, all four wheels, with speed bleeders, one at a time. I got a high, but fairly soft pedal that will eventually stop the car, but too dangerous in traffic. I'll have to run another quart through. This time, I think I'll open all four speed bleeders with a hose on each into a collecting jar. I've got so much Dot 3 on me this weekend my wife thinks it's a new brand of aftershave. Fortuneately, my Cobra is not painted, or I'll have to be doing a re-paint.

[Dwight]

GreenSnakeDoc, bleed them one at a time. You can not keep enough fluid in the master cylinder to feed all four at the same time. If you have brake fluid on your un-painted body, you will need to remove all traces of the fluid before you paint. Brake fluid is very bad on paint, steel, humans, just about everything.


I start with the rear passenger side because it is the farthest from the master cylinder. I use a short piece (2" + -) of clear tubing that will fit (hold) on the bleed valve. I insert the small tube into a larger one so the fluid will flow easier. I drill a hole into the top of a 1 liter plastic bottle and insert the tube. Then I top off the reservoir with fluid. I open the bleeder valve and let gravity do the rest. When I see it draining with no air in the fluid, I close the valve and go to the driver side and repeat the process. Don't forget to add more fluid to the MC each time. Then to the front passenger side and then the last one that is the closest to the MC. Takes 45 minutes to an hour but I can do it myself.
I have done this on several different Cobras. It works everytime.
Dwight

P.S. I'm not an expert! At anything!

[Maurice Butler]

if you clamp the flexable hose on the ones you haven't bleed you will speed the process up as well