Just finished rear disc swap,now pedal goes to the floor.Why?

[felix5oh]

I did everything they said to do on several sites and when the car is on the pedal sinks to the floor. I'm afraid to leave my street like that to do the 40 MPH test. The idle goes up when the pedal is pressed too. When the car is off the pedal is in the middle and feels normal. It's acting like the booster is leaking air but it was fine before this conversion.

Lets review:
Hardware is installed correctly and the e-brake works fine.
The soft lines and banjo bolts had to be modified to go from the calipers to the existing lines.
Removed the guts from the front part of the proportioning valve and cap it.
Installed the new Willwood proportioning valve in the proper location and it's wide open right now.
Bleed the brakes. Several times. There is fluid in the calipers.

Help me out guys. Am I missing a step? What did I do wrong?
I'm going to find a brake shop that can make a hard line from the caliper to the existing hard lines. But that won't cause the pedal to go to the floor

[wraithracing]

If your pedal is going to the floor you have a hydraulic issue somewhere. Either you have air in the lines and need additional bleeding to remove the air or you are leaking fluid somewhere.

It's possible depending on how you did the install and the bleeding of the brakes that you over extended the rod in the Master Cylinder and now your seals are blown and the M/C is leaking internally and not building pressure when you depress the brakes.

The Booster affects the amount of assist to the M/C. If the Booster is leaking air, then your pedal would normally be harder to press not easier. This is the reason why when then car is off the pedal feels "normal" but is easier to press when the car is running. Most likely the engine idle is jumping up when you depress the brakes all the way down you are using all the vacuum in the booster and the drop in vacuum at the engine is causing a surge in the idle to compensate.

When you bleed the brakes are you getting good fluid flow from each bleeder screw? The fluid should be forced out not just leak or dribble out. If the fluid doesn't come out with some force or pressure behind it, you need to look at the M/C and your lines. The M/C first to verify that it's not blown as I mentioned above and the lines to make sure you don't have a collapsed line that isn't allow flow and the bleeding of air in the system. Also verify that all of your bleeder screws are facing up on the calipers. If the screws are facing down, then the air can't be bleed from the system and the calipers are flipped and need to be changed too.

Check that out and let us know what you find.

Trey

[Mike Croke]

When you installed the rear discs, which master cylinder did you install?

[felix5oh]

I work out of town so I won't see the car until next weekend but I will check the M/C first.
On the rear right caliper the fluid came out much faster than the left one but found no restrictions. I want a shop to make me a hard line from the caliper the the joint behind the axle. I dont like the soft line into hard line joint, I just want a cleaner look and free flowing fluid. I was also thinking there might be air trapped in the old proportioning valve. I've never had this type of problem with brakes. I was afraid of this happening when I started the swap. Thanks for the input.
My car is a 1985 GT and the M/C is stock.

[wraithracing]

Did you gut the old proportioning valve?

Did you install an adjustable proportioning valve?

I still think you may have a M/C issue or air in the system, but you will need to do both of the items above to be able to use the rear disc as intended. The OEM proportioning valve is setup of rear drum and is not correct for a rear disc setup.

I understand the working out of town myself. Let us know what you determine.

Trey

[Mike Croke]

My car is a 1985 GT and the M/C is stock.

I'm guessing that be the problem. The stock master cylinder is sized to supply fluid for the rear wheel cylinders, not for rear calipers. When switching to rear discs, the master cylinder has to be switched as well.

[Jack Hidley]

If the engine idle changes when the brake pedal is depressed something is really wrong. Either the brake booster is broken or the cruise control vacuum valve that goes to the brake pedal is messed up. Normally if either of these conditions cause a vacuum leak to get to the engine, you will be able to hear it from under the dash.

Exactly which rear brakes (year and application of the rotor and caliper) are installed in the rear of the car? Without this information, there is no way to know if the car has the correct m/c or not.

Exactly which front brakes are on the car?

You DO NOT want to connect a hard metal brake line between the axle and brake caliper, if you have an OEM style floating single piston caliper. Floating calipers MUST be able to move side to side as the brake pedal is depressed and as the brake pads wear. With a solid metal brake line between the caliper and axle housing, the brake line is being bent back and forth every time the brake pedal is depressed. The brake line will eventually fatigue and crack.

[felix5oh]

I'll come up with a diffrent hard to soft line setup. Correct me please but, I thought the 79-85 cast iron MC has a larger orface and is OK for this application. What MC should it be?

Yes and yes the proportioning valve is gutted and an adjustable one is in the correct place. There's fluid in the entire line. I will check the booster and step back and inspect every part of the system this Saturday when I have the time. There might be a restriction or there may be air in the old PV. I'm just not sure but I'm going to find out.

[wraithracing]

As Jack stated, the MC might be fine for your application, but without more information as to what you have actually installed there is no way to tell for sure.

If you can give us the full information as to what you have installed that will help to determine where your issue(s) may be. I am willing to bet that one of us here has installed a setup similar to yours and will be able to help with your problems. Also you have the opportunity to pick the brain of Jack who has had the opportunity and good fortune of seeing and doing more than the average Mustang owner thanks to his day job with Maximum Motorsports. I know he has helped Me many times and I always value his advice.

Trey

[felix5oh]

Turbo Coupe/93 Cobra calipers and discs, North Racecars bracket. Softlines to a flare hardline on the existing 1993 8.8 axle. The body is a 1985. Willwood PV where the joint was on the passenger side under the hood.

The banjo bolt seems almost oversized for the softlines. I don't like it. I would like to know what has worked and I will rebuild all those lines. Help please.

[Jack Hidley]

Assuming that your front and rear brakes are from a 1993 Mustang Cobra, the front pistons are 60mm and the rear pistons are 45mm. You should more or less duplicate the rest of the parts in this brake system.

Your car currently has a 21mm (0.827") diameter m/c. The 1993 Mustang Cobra has a 1" diameter m/c. This is an increase in area of 46%. This means that the brake pedal will have 46% less travel and require 46% more force for a given deceleration, if you install the 1993 Mustang Cobra m/c.

However the 1993 Mustang Cobra also had a 205mm dual diaphragm booster compared to your 220mm single diaphragm booster. This will provide a lot more assist to help compensate for the 46% increase in pedal effort. You can get a rebuilt 1993 Mustang Cobra booster from Latemodel Restoration Supply. If you install the 1993 Mustang Cobra m/c, I would also do it with the matching booster.

If you don't want to spend the money on the 1993 Mustang Cobra booster, you can install a 1986-93 152mm dual diaphragm booster with a 1994-95 Mustang Cobra 15/16" m/c.

In both cases, you will need to install the matching m/c installation kits for your car. We have those available on the MM website.

Your rear calipers have M10 fluid bolts on the fluid inlet. The hole in the end of the brake hoses should be slightly larger than this (about 0.40" ID). There should only be a small amount of play. We sell braided SS hoses for this application also, if your current hoses won't work properly.

Also if your brake booster is leaking vacuum, the most common place for this to happen is under the dash, right where the booster pushrod comes through the firewall. There is a filter around the pushrod here. This is where the booster draws air in to replace the vacuum when the pedal is pressed.

[Mikestang]

^ Smart guy

[wraithracing]

All great information by Jack as usual.

I will also mention that if you car was equipped with cruise control, the vacuum switch on the brake pedal that deactivates the cruise can be a source of vacuum leaks too. It usually only happens when the pedal is depressed, so check that too if you car is so equipped.

Good Luck!

Trey

[felix5oh]

Thank you Jack, the front brakes are stock. I read somewhere my MC was good if I was only doing the rear. I will order the 1993 Cobra MC and booster for next weeks work. I was not planning on changing the front brakes. I DO need the braided line from the rear caliper to connect to my existing hard line.

[felix5oh]

Thank god for Maximum Motorsports for doing all the guess work for me. Just ordered MMBK7P
This weekend I will inspect the booster and MC but I think my problem is in the rear hardline to softline to banjo bolt.

I knew I would love it here at Four Eyed Pride. I asked for help and didn't get made fun of for not knowing the right combo. Thank you for helping.
My car is a 1985 GT convt. originally with auto trans and CFI and 7.5 axle and now has a 5 speed and carburator and an 8.8 axle from a 93 GT.
I bought it in 1996 and I'm the 2nd owner. I have a 1986 roller block sitting on a stand waiting to replace the flat tappet H.O. 302 block someday. I need to get these brakes right to move on to the next step, the interior and guages. Thank you FEP.

[wraithracing]

Glad we were able to help out with your issues. Hopefully you can get everything working correctly and enjoy driving your GT!

I will also say that I have the 93 Cobra M/C & Booster setup on my 79 Pace Car along with 10th Anny Cobra front & rear brakes including the IRS. I have to say the pedal feeling is firm and honestly not my favorite. I know my setup is different than yours, but I will say that the 93 Cobra booster and M/C are not always the best answer to a brake problem. I would recommend getting the brakes working effectively first and then decide if you need to make a M/C or booster change. I previously had a 85 GT with a similar setup to yours and used the stock booster with the SVO M/C. Although looking back now I might have used a different M/C the booster worked just fine and gave plenty of assist.

Another point to make is that the 93 booster generally takes some beating and pounding on the driver's side strut tower for clearance. Every car is a bit different, but either way I have installed the setup in 2 Four Eyes and I have no plans to ever do it again.

Trey

[whyidontknow]

Not to hijack but after reading this I have a question. Is there advantages to just going to a manual brake setup when running the newer disc brakes?

[wraithracing]

Not to hijack but after reading this I have a question. Is there advantages to just going to a manual brake setup when running the newer disc brakes?

For most street cars and road racers I would say no. Going from power brakes to manual brakes is not a simple matter of removing the booster. The pedal is actually different between the manual and the power brakes, so in order to do the swap properly you have to change the brake pedal in order to have the correct pedal ratio. Here is a link to the MM page of manual swap kits.
http://www.maximummotorsports.com/Mu...ions-C244.aspx

There are times and reasons for going to manual brakes, but in most street vehicles I don't think there's much benefit. If you have engine clearance issues with a booster, low vacuum issues, or you just want the lack of a booster in the engine bay for aesthetics, then you might consider going manual.

Hope that helps!

Trey

[Jack Hidley]

Felix,

The MMBK7P is designed for Mustangs that came with an 8.8" axle assembly. These cars have the termination of the rear brake hard line on the floorpan just to the left of the differential housing. If you haven't moved the hardline to this location, the MMBK7P won't fit.

On a street car, there is no performance advantage to installing manual brakes. They do give more room in the engine compartment and won't be susceptible to engine vacuum variations. We don't recommend installing them on a street car unless there is some overwhelming reason to do so.

[Dean_T]

Another aspect of the non-power brake pedal is it rides higher than the power brake pedal. When I first started driving my '78 2dr sedan I initially hit the side of the pedal with my shoe until I learned to raise my foot just a little higher to clear the pedal to use the brake. I imagine this would make heel and toe harder to do.

[felix5oh]

Thanks Jack, I had already modified the brake line to come to the center to accept the 8.8 axle 17 years ago when I did the axle swap from the 7.5
I'm going there this morning to work on the car and let you know what I found.

[whyidontknow]

For the responses to manual brakes thank you all.

[Bungy]

Got any updates on this? Did you find the problem?

[PIGBOY]

Maximum smarty pants sports dot com = right on info

[felix5oh]

So I installed the hard lines and soft lines in the kit this weekend. I also took off the MC and inspected and cleaned it. It pumps and is not overextended and slides in and out like a normal one should. Inspected the booster and didn't see anything strange. Bleed the breaks for the last time until clear fluid was there. Test drove the car and the pedal is not to the floor but is still way down close to the floor. I couldn't make it lock up from a 35 MPH stab. The Willwood manual PV is wide open.(turned in all the way) The booster is OK and doesn't change the idle. I feel the pedal should be in the middle somewhere. Maybe I do need to buy that Cobra MC and booster to make it happen=(didn't want to do that)

My one stupid question:
Before the brakes were bled I fabricated a bracket to fit the newer style e-brake cables and I pulled it to test the action. "Does this affect the fluid in the calipers when the cable is pulled before the fluid pushes the caliper pistons???
The e-brake was off while bleeding the brakes.