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Old 10-08-2009, 06:32 PM   #1 (permalink)
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Brakes Wont Hold

My car will push through the line like with street tires, a slow roll but still not holding.

Its a 67 coupe that I switched over to 68 single piston discs. The brake system has been totally replaced.
New 68- early 70 Man/Pwr disc MC with a one inch bore
New hard and soft brake lines on the front and rears
New calipers up front, new wheel cylinders in the back
I ditched the factory combo valve and put an SSBC combo/prop valve in its place. The Hurst Line loc is between the combo/prop and front brakes.
System has been bled and re bled, its clear.
Use DOT 3 fluid
Fordnut did the same swap, except he used a straight prop valve and he says his works good and holds the car with the line lock.

They stop ok, not great. I have not really cranked them to see if they will lock up. Good pedal, about 1/3 travel, then firms up, and the pedal is solid after that, no leak down or mushing.

The only thing not replaced was rotors/pads, came with the car when I bought it. The rotors look good, they have a lot of meat, are in great shape, not glazed (pics posted on my making progress thread, 67 Mustang coupe). The pads look brand new. Looks like any normal wear pattern on a disc break rotor, cant see anything from that.

No pulling, car does stop in a straight line. I even engaged and held the line loc at low speed once with a minimal pres to see if it would pull one way or the other, it doesnt and did slow the car down a lot.

So what am I missing. I am thinking my next step would be a press test on the front calipers, would have to buy a press test kit. Let me know if I am over looking something stupid here.

Yes, I went back and checked my plumbing, its got the right tubes to the righ bowl outlet to the right inlet/outlet on the prop block (SSBC marks those pretty well).

Thanks
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Old 10-08-2009, 07:02 PM   #2 (permalink)
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Re: Brakes Wont Hold

My brother had the same complaint until I told him to push the pedal real hard when setting the line loc. This was in a Chebbie and that did it for him. I have to do that too--just like a panic stop I push the pedal hard.

I assume that your car stops normally when not using the line loc.

When my first line loc went bad after a year of use, it too acted the same way and would roll out during a burnout. I replaced it and the system works great again. All it takes is a little dirt in the brake system or a bad seal to mess it up.
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Last edited by dennis111; 10-09-2009 at 04:02 AM.
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Old 10-09-2009, 04:31 AM   #3 (permalink)
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Re: Brakes Wont Hold

Thanks Dennis, the car stops OK, not sure if it will lock up anything havent tried the panic stop.
I give it a few good pumps, then hit the button and give it a couple more pumps, when the loc is engaged the pedal does get rock hard and high.

Thanks, Chris
Quote:
Originally Posted by dennis111 View Post
My brother had the same complaint until I told him to push the pedal real hard when setting the line loc. This was in a Chebbie and that did it for him. I have to do that too--just like a panic stop I push the pedal hard.

I assume that your car stops normally when not using the line loc.

When my first line loc went bad after a year of use, it too acted the same way and would roll out during a burnout. I replaced it and the system works great again. All it takes is a little dirt in the brake system or a bad seal to mess it up.
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Shortened & Mini Tubbed. Strange Nodular 9, Spooled 4:56, Mark Williams Axles.
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Old 10-09-2009, 05:09 AM   #4 (permalink)
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Re: Brakes Wont Hold

It it possible you have the pressure biased to the rear brakes? Have you tried adjusting the proportioning valve??
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Old 10-09-2009, 08:04 AM   #5 (permalink)
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Re: Brakes Wont Hold

I kept decreasing the rear biased, braking the car, and decreasing until I could feel the biased move to the front. I did a panic stop today, and the rear barely skidded, so I took another turn out.
I think I am onto it though, got under the car with a spot light, on the driver front caliper I saw a bit of wettness between the banjo fitting and the caliper frame. Hard to see, missed it before. Tightened it down, did a quick bleed with the Vac Pump, and it made a huge difference. The car still pushed through but not much, braked a lot better. Checked it again and its wet again so still not holding press at the calipre. I am going to swap out the coppper washers and see if it will fix it.
Like my first post, something stupid, but there was no visible loss of fluid and its not dripping, its just a little wet around the washer. Doesnt take much I guess.
Quote:
Originally Posted by 68fastang View Post
It it possible you have the pressure biased to the rear brakes? Have you tried adjusting the proportioning valve??
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Old 10-09-2009, 08:07 AM   #6 (permalink)
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Re: Brakes Wont Hold

Interesting info and questions guys. I need to install a line loc this winter, what brand do you recommend ?
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Old 10-09-2009, 08:14 AM   #7 (permalink)
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Re: Brakes Wont Hold

I don't think those disc brakes are up to the job as well as a set of drums would be.

The drums setup grabs almost 40 SQUARE inches of drum, where as the disc setup only grabs about 14 square inches...

I know I know, brake fade Etc... How many run's are you allowed to make in a 5 minute period? Or a half hour for that matter? Brake fade only comes into play when the brakes are hot...

But that's just me

You can always change the rate of compression that your brake pedal puts on the master cylinder to increase your force applied to the brakes.


FE
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Old 10-09-2009, 08:43 AM   #8 (permalink)
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Re: Brakes Wont Hold

How you change compression, bigger bore on the MC? Thanks.
Quote:
Originally Posted by FEandGoingBroke View Post
I don't think those disc brakes are up to the job as well as a set of drums would be.

The drums setup grabs almost 40 SQUARE inches of drum, where as the disc setup only grabs about 14 square inches...

I know I know, brake fade Etc... How many run's are you allowed to make in a 5 minute period? Or a half hour for that matter? Brake fade only comes into play when the brakes are hot...

But that's just me

You can always change the rate of compression that your brake pedal puts on the master cylinder to increase your force applied to the brakes.


FE
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521 BBF-Solid Roller -Kaase P51 Heads-Victor-AED Dominator - Jerico DR4, Hurst V-Gate.
Shortened & Mini Tubbed. Strange Nodular 9, Spooled 4:56, Mark Williams Axles.
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Old 10-09-2009, 09:08 AM   #9 (permalink)
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Re: Brakes Wont Hold

I would better look into the angle of compression that you get with the brake pedal.

The closer the Master pushrod sits to the Pivot point of the brake pedal, the greater the pressure delivered to the brakes.

Improve that point and improve your clamping force.
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Old 10-09-2009, 09:44 AM   #10 (permalink)
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Re: Brakes Wont Hold

Ahhh Hah. Its all stock stuff, and in a direct line, level pushrod from the pedal to the MC now? Am I reading you right or did I miss this.

Thanks
Quote:
Originally Posted by FEandGoingBroke View Post
I would better look into the angle of compression that you get with the brake pedal.

The closer the Master pushrod sits to the Pivot point of the brake pedal, the greater the pressure delivered to the brakes.

Improve that point and improve your clamping force.
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521 BBF-Solid Roller -Kaase P51 Heads-Victor-AED Dominator - Jerico DR4, Hurst V-Gate.
Shortened & Mini Tubbed. Strange Nodular 9, Spooled 4:56, Mark Williams Axles.
"old School - WWDD"
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Old 10-09-2009, 09:53 AM   #11 (permalink)
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Re: Brakes Wont Hold

Quote:
Originally Posted by FEandGoingBroke View Post
I don't think those disc brakes are up to the job as well as a set of drums would be.

The drums setup grabs almost 40 SQUARE inches of drum, where as the disc setup only grabs about 14 square inches...

I know I know, brake fade Etc... How many run's are you allowed to make in a 5 minute period? Or a half hour for that matter? Brake fade only comes into play when the brakes are hot...

But that's just me

You can always change the rate of compression that your brake pedal puts on the master cylinder to increase your force applied to the brakes.


FE
I did this swap recently on my 68 as Gydyup said..The discs hold much better than my drums ever did..Actually I used to have the same problem when I had the drums of the car not holding with the line loc..I also had someone cut me off this summer and had to hit the brakes very hard..It locked up the front tires and actually shocked me....As for brake fade you can get that in a single pass at the drag strip trying to get stopped from 120mph or so..It's also nice to not have the car pulling to one side or the other as the drum brakes often do..
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Last edited by frdnut; 10-09-2009 at 09:55 AM.
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Old 10-09-2009, 10:02 AM   #12 (permalink)
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Re: Brakes Wont Hold

I have the SSBC front discs and a Hurst Roll Controll. I pump brakes two or three times with the last pump being very hard, press the button and then pump the brakes again. This is the only way I could get it to hold everytime.
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Old 10-09-2009, 10:58 AM   #13 (permalink)
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Re: Brakes Wont Hold

Quote:
Originally Posted by Gydyup View Post
Ahhh Hah. Its all stock stuff, and in a direct line, level pushrod from the pedal to the MC now? Am I reading you right or did I miss this.

Thanks
That's what I was referring to yes.


Also, I understand Frdnuts thoughts, but ummmmm, don't we have about 80 years of drag racing history where 90% of it was with drum brakes? There are still many that use drum brakes. Also, do tracks ban you from having a hella fast car with drum brakes? I don't know...

I do know that the increased pressure available to the brakes by simply moving the pushrod 1/2 inch will be tremendous.
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Old 10-10-2009, 06:54 AM   #14 (permalink)
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Re: Brakes Wont Hold

Thanks, I have the Hurst as well, once you hit the button does your pedal get rock hard, that last pump seems sort of moot, wondering what type of press it is carrying at that point.
Disc, Drums, one thing I keep scratching my head on re holding power on a burnout. I recall back in the good old days in 1982- my 68 fast back, iron head small block 4 speed, went 13:40s on Drag Master poly glass tires, I would dump the clutch, get on the drum brake pedal and do a nice smokey burnout - poor kids line lock. Heating up those poly tires was important.
Quote:
Originally Posted by cahorne View Post
I have the SSBC front discs and a Hurst Roll Controll. I pump brakes two or three times with the last pump being very hard, press the button and then pump the brakes again. This is the only way I could get it to hold everytime.
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521 BBF-Solid Roller -Kaase P51 Heads-Victor-AED Dominator - Jerico DR4, Hurst V-Gate.
Shortened & Mini Tubbed. Strange Nodular 9, Spooled 4:56, Mark Williams Axles.
"old School - WWDD"
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Old 10-10-2009, 09:32 AM   #15 (permalink)
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Re: Brakes Wont Hold

Quote:
Originally Posted by FEandGoingBroke View Post
That's what I was referring to yes.


Also, I understand Frdnuts thoughts, but ummmmm, don't we have about 80 years of drag racing history where 90% of it was with drum brakes? There are still many that use drum brakes. Also, do tracks ban you from having a hella fast car with drum brakes? I don't know...

.
I guess it all depends on the tracks shutdown area..Some are very short and lead right into a metal guard rail or a bunch of trees!..I have never heard of a track banning you for having drum brakes but its up to the driver to know if the car will stop or not....With my old 302 combination I was going through the traps at about 101 mph..I had to always use the second return road and brake pretty heavily to make that one.....I know adding roughly 20mph as I hope to with my new engine I would never make it..
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