Soft Brake Pedal (Circle Track Mustang)

[jdwest711]

Let me explain the brake system on my car.

 

Originally all Drums, manual brakes

 

Now

 

- Manual brakes

- 74 Maverick Master Cyl for Manual Disc/Drums (AutoZone)

- Proportioning valve from Disc/Drum Mustang donor car.

- Fatman 2.5" drop spindles which require:

  -- 78 Granada Rotors/Bearings/Seals/Caps (Rockauto)

  -- 78 Camaro Calipers (from Camaro Donor Car) (I hated doing that)

  -- Carbon Metallic Brake Pads (AutoZone)

- Rear Shoes (AutoZone)

- Rear Brakes are stock 9" 10"x2"

 

I've blead it 100 times

Bleed ports are on top

 

My pedal is still way too soft.  It feels like my rear brakes are working harder than my front brakes. Any suggestions?

Do I need a residual valve for the rear brakes?

The front brake lines are from a donor car too.  I'm going to replace them because weak/swelling lines can cause a soft pedal but... to me it seems more than that.

 

Any suggestions are greatly appreciated!

 

[Jeff73Mach1]

Have you tested pressure at the calipers?

 

[jdwest711]

Have you tested pressure at the calipers?

No, I have never done that.  I'm assuming they make a gauge for that, which connects to the brake line?  Any idea how much pressure it's supposed to be?

 

[Jeff73Mach1]

The gauge usually mounts where your bleeders go. Pressure for front brakes varies and with your combo it is hard to say, but I would think 1400psi up front and 650-750 on back brakes would be close.

I used this when I was having some issues with my set up.

https://www.summitracing.com/parts/ssb-a1704?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-ssbc&gclid=EAIaIQobChMIpqCd_-WX2wIViC-BCh093QBdEAQYAiABEgIj5_D_BwE

Master cylinder piston bore size is pretty critical in getting the proper feel. You my want to go smaller to increase pressure and get a firmer pedal, but a guage and testing would be where I would start.

I went back to semi metallic pads and my braking feels much better than with race type ceramic pads

 

[jdwest711]

The gauge usually mounts where your bleeders go.  Pressure for front brakes varies and with your combo it is hard to say, but I would think 1400psi up front and 650-750 on back brakes would be close.

I used this when I was having some issues with my set up.  

https://www.summitracing.com/parts/ssb-a1704?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-ssbc&gclid=EAIaIQobChMIpqCd_-WX2wIViC-BCh093QBdEAQYAiABEgIj5_D_BwE

Master cylinder piston bore size is pretty critical in getting the proper feel.  You my want to go smaller to increase pressure and get a firmer pedal, but a guage and testing would be where I would start.

I went back to semi metallic pads and my braking feels much better than with race type ceramic pads

That will definitely take out the guessing game, I will definitely try that.  You also said something else that made a lot of sense.  I have not raced on the Carbon metallic pads yet, I only tested them out in my driveway.  I have been told that they need to heat up a bit before they really start to work good.  Maybe that is also what I'm feeling.

 

Thanks for the tip!  Now to buy another tool I don ‘t have.

 

[midlife]

If you're feeling a soft pedal only in your driveway, you might want to adjust your rear brakes such that the rear wheels do not spin more than 1/2 turn with no brakes applied. That puts the adjusting wheel about where it should be for the rears, and with manual brakes, you'll have a firm pedal right off the bat.

 

[Bentworker]

2nd on Midlife's post.

Drums require adjustment, especially since nobody profiles brake shoes to the drum diameter anymore.

 

[jdwest711]

If you're feeling a soft pedal only in your driveway, you might want to adjust your rear brakes such that the rear wheels do not spin more than 1/2 turn with no brakes applied.  That puts the adjusting wheel about where it should be for the rears, and with manual brakes, you'll have a firm pedal right off the bat.

I have adjusted the rear brakes twice. I don't think that is it. I'm going to try and adjust them again but I'm also thinking about installing a 10lb Residual Pressure Valve. I think mine was removed when I swapped Master Cylinders. The all Drum M/C probably had it inside the fitting.

Also, it's not that I only feel it in my drive way. I haven't raced with these pads yet. I'm assuming it's going to brake the same but I don't know. They might grab better when they get some heat into them. At least that is what I've been told.

Thanks for the advice!

 

[Jeff73Mach1]

You probably don't need a residual pressure valve. Generally not necessary with a high mounted master cylinder.

 

[Don C]

What method did you use to bleed your brakes?

I would recommend an adjustable proportioning valve.

 

[Hemikiller]

My thought is the diameter of the 78 Camaro piston vs the 74 Maverick. Out of curiosity, I looked up the calipers for both vehicles on RockAuto. The Centric parts show some dims of the caliper. The Maverick has a 66mm bore, while the Camaro has a 75mm. If the diameter of the piston is too large, you'll get very little force applied to the piston, as the MC is running out of volume. My thought would be to use a MC with a larger bore for the fronts to get more volume to the calipers.

Camaro caliper

https://www.rockauto.com/en/moreinfo.php?pk=2036676&cc=1035188&jsn=711

Maverick caliper

https://www.rockauto.com/en/moreinfo.php?pk=2053268&cc=1132210&jsn=468

 

[jdwest711]

What method did you use to bleed your brakes?

I would recommend an adjustable proportioning valve.

I used 2 different methods.  The old fashion 2 person method and the 1 person method, with a vacuum line and bottle.  Both have work for me in the past.  I'm not new to turning a wrench or working on brakes.  This is an issue because of modifications I've done so it's a little out of the ordinary.  I'm sure it's not my bleeding techniques.

 

I have an adjustable valve for the rear brakes but i haven't installed it yet.  I don't consider that a fix to this issues so I don't want to install it until I get the pedal right.

 

[jdwest711]

My thought is the diameter of the 78 Camaro piston vs the 74 Maverick.  Out of curiosity, I looked up the calipers for both vehicles on RockAuto. The Centric parts show some dims of the caliper. The Maverick has a 66mm bore, while the Camaro has a 75mm. If the diameter of the piston is too large, you'll get very little force applied to the piston, as the MC is running out of volume. My thought would be to use a MC with a larger bore for the fronts to get more volume to the calipers.

Camaro caliper

https://www.rockauto.com/en/moreinfo.php?pk=2036676&cc=1035188&jsn=711

Maverick caliper

https://www.rockauto.com/en/moreinfo.php?pk=2053268&cc=1132210&jsn=468

Now that's interesting!  Thanks for doing that research.  I am going to start looking at Spec for M/C to see if I can get a bigger one to mount up right.

Thanks!!!

 

[Carolina_Mountain_Mustangs]

Do not know what piston size is in the master cylinder you used. Original was 15/16" for power brakes and 1" manual.

If the bleed valve is not at the highest point you will not get the air out. If it is not highest point you can take the caliper off and rotate until the bleeder is highest point put something between pads and bleed and put back on.

I believe one statement was incorrect. A larger diameter piston in caliper applies more pressure to the pads not less due to surface area increase. The amount of fluid needed to apply the disc is minimal since they do not retract and drag on the disc.

I am curious as why you went to disc. Drum has less drag than disc. Unless you are having to brake a lot I would prefer drum on a circle track car to get the little bit less drag. Would also weigh the parts and see which set up has the least un sprung weight to carry. If running a road course you would want disc to prevent fade. What length races do you run in? 20 lap heats 50 event? Sometimes here a really long race will be 200 laps then brakes are very important.

David

 

[jdwest711]

You probably don't need a residual pressure valve.  Generally not necessary with a high mounted master cylinder.

From what I’ve researched, drum brakes always need a 10lb residual valve to compensate a little for the spring tension.  I think I had one in my old drum/drum M/C but it got removed when I replaced the M/C.  Low mounted M/C’s require a 2lb residual valve the maintain a little pressure even on disc brakes.

 

[jdwest711]

Do not know what piston size is in the master cylinder you used. Original was 15/16" for power brakes and 1" manual.

If the bleed valve is not at the highest point you will not get the air out. If it is not highest point you can take the caliper off and rotate until the bleeder is highest point put something between pads and bleed and put back on.

I believe one statement was incorrect. A larger diameter piston in caliper applies more pressure to the pads not less due to surface area increase. The amount of fluid needed to apply the disc is minimal since they do not retract and drag on the disc.

I am curious as why you went to disc. Drum has less drag than disc. Unless you are having to brake a lot I would prefer drum on a circle track car to get the little bit less drag. Would also weigh the parts and see which set up has the least un sprung weight to carry. If running a road course you would want disc to prevent fade. What length races do you run in? 20 lap heats 50 event? Sometimes here a really long race will be 200 laps then brakes are very important.

David

Fatman 2.5” drop spindle conversion requires Granada rotors and Camaro caliper.  I do brake very hard driving into the turns, this isn't a drag car.  Races are usually 25 laps long, so I'm pounding on the brakes at least 50 times a race.

 

As mention in my first post, bleeders are at the highest point.

 

I'm definitely going to compare the size of the M/C for a Camaro and Maverick.  When I find that information I will decide then if I need to upgrade my M/C.  My first attempt is going to be readjusting the rear brakes and a 10lb residual valve.

 

Thanks for the input!

 

[Don C]

The 2 person method is best for getting rid of air in the lines. Using a hand operated vacuum pump at the wheels may not move the brake fluid fast enough to pull the air out of high points, what is called an inverted "U" is especially hard to get air out of. From reading your posts I assumed you had experience with wrenches, which is why I didn't ask if you had bled the master cylinder before installing it.