Members with 4 wheel disc systems.....Can you get the tires to skid?

[1 Wild R/T]

But does it work???? Turned out pretty good for a quick fix... Bridgeport is your friend..

 

[eldubb440]

I remember pulling a booster out of a C body, it looked exactly like the one in my GTX........I soon discovered that rod ^^^^ was a different length, most likely too long; this was likely 1986........ anyway, I cut and welded it to the proper length and it worked. I still have that booster on my shelf

 

[Kern Dog]

But does it work???? Turned out pretty good for a quick fix... Bridgeport is your friend..

I felt unusually tired and sleepy today. I went with the wife to get a displaced dog for a friend and didn't make it out back to do any car stuff today.

 

[Sixpactogo]

The helpful FBBO member had a better idea. (I'm keeping his name a secret until he says it is okay to tell.)


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Yes! An adjustable pushrod. This was a stock unit modified to allow extension.

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I could put a nut on the threaded section so the two halves would still make contact and make a solid connection. My rough guess is that 1/4" of adjusting out may be more than enough to bring the pedal up to where I want it.

Looks like a great solution to me.....Once you get it adjusted to bring your pedal up to the clutch level, You could weld it and file a groove for the anti rattle spring if you put one back in.

 

[1 Wild R/T]

Looks like a great solution to me.....Once you get it adjusted to bring your pedal up to the clutch level, You could weld it and file a groove for the anti rattle spring if you put one back in.

I don't see a need to weld it.. Once it's bolted at both ends it can't unscrew... It's only .060 smaller in diameter with the bolt than it was with just the reduced area for the spring, you can still pop a spring into the area where the bolt lives....

 

[Kern Dog]

Mixed results.
I swapped in the aluminum 4 bolt 1 1/32” master cylinder.

I bled it out using the Motive bleeder. At first it felt okay, an acceptable amount of pedal travel. Out on the road it stopped okay. I think I like the smaller 15/16” unit more.
On the way back home the pedal wouldn’t hold at a stop light. While driving it seems to stop well but a sinking pedal makes me suspect a bad master cylinder. I see no leaks. I’ll probably bleed and retest. I wanted to put the pushrod in last… as a sort of cherry on top of all of this.

 

[1 Wild R/T]

Well that's a bummer...

 

[Kern Dog]

I still have the aluminum 2 bolt 15/16" and the iron 4 bolt.
The only thing this costs me is time and effort. I'm a bit annoyed with it though.

 

[eldubb440]

I remember pulling a booster out of a C body, it looked exactly like the one in my GTX........I soon discovered that rod ^^^^ was a different length, most likely too long; this was likely 1986........ anyway, I cut and welded it to the proper length and it worked. I still have that booster on my shelf

my bad, I cut/welded the rod coming out of the booster; not that it matters here, just wanted to correct myself........ and 37 years later I barely remember the details...... great thread, carry on

 

[Kern Dog]

I may have good news, finally.
No, I don't know if I'm able to skid on the road but it sure locks them up on the decomposed granite. Driving around the backyard, they feel fantastic.
It was a frustrating day though but I'm the only one to blame for it.
First off, I pulled the iron 15/16" MC....

Then swapped in the new aluminum 1 1/32" unit....

I bled the system and went for a drive. The brakes were okay but not as grippy as with the smaller MC. On the ride back, the pedal started sinking to the floor at stop lights. I suspected that the master cylinder was bad so at home, I pulled and boxed it up, then put my 2 bolt aluminum 15/16" unit back in...

In the shop, even after bleeding, the 15/16" had good pedal feel but even without driving it, just sitting in the car at rest the pedal was firm but slowly sunk like before.
What the heck???
I saw no leaks at the calipers or anywhere else. I had Mary press the pedal while I looked. Somehow I missed it...It was leaking from the fitting at this port:

The fittings are seasoned ones...originals from the mid 70s. When I get around to replacing the brake lines, I'll look for new fittings. In the meantime, I did change the brake pedal pushrod.....Stock type on top, modified one below:

I used washers as spacers and coated the threads with Locktite, then swapped it in.

The pedals are almost even now, close enough for me.

I could adjust the clutch pedal if I wanted but I might leave it as is.

The pushrod clears everything.

Man....it really is busy up under here:

 

[68 Sport Satellite]

Excellent photos KD. Can't wait for you to fix the leak and re-test the aluminum MC.

 

[Kern Dog]

Oh, I did. The leak is gone, the system is bled and it stops like a new car.....at least at slow speeds. It has been my experience (with this car at least) that if it stops good at slow speeds, it stops as good or better out on the road. I'll have it out on the road tomorrow and report back.
I have to change rear wheel cylinders and a master cylinder in this car:

It is a traditional front disc, rear drum, manual master cylinder setup.

 

[Kern Dog]

***UPDATE***
I have found a combination that makes enough force to skid the tires.
No, it was not on wet pavement and not on oily roads!
This setup seems to be the BEST that I have had in all the years with the car. The pedal feel is great, pedal travel is not too long or too short, the stopping ability is fantastic and twice I got the front to skid. For some reason, the left seemed to want to skid more than the right. The front tires are ready to be replaced and I have a spare set so the possible "flat-spotting" of the tires isn't an issue.
I'm curious if there is any possibility that the side with the longest brake line could be the one most likely to get less or at least delayed pressure?
I know very little about hydraulic function so this is one of those things that have me curious.
Regardless.....
This is great news.
Now, I have to go back and REadjust the brake light switch. I did have it adjusted but it seems to have either moved a little or needed more attention.
SCORE !

 

[BergmanAutoCraft]

I am having a hard time making sense of my brakes. I've tried multiple combinations and no matter what I do or try, I can't get them to skid on dry pavement.
I'm not a trained mechanic but I do have years of experience with these machines. One method that I use is the process of elimination. If the system has faults and you change enough parts, eventually you will find the problem. This is not easy on the wallet but sometimes it is the only course of action you have to work with.

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This car stops well but it isn't awe inspiring. I never feel like I need to lift from the brake pedal because I am about to skid or stopping faster than I want. I have good parts in the car but something just isn't right.
Originally, it was a 4 wheel 10" drum system. You all know how those perform.
My first change was an A body power booster and 11" front discs. It stopped well and never had me feeling like I was scared to drive it fast.
Later, I upsized to the Cordoba 11.75" front rotors.
In 2006 I pulled the rear drums and installed the Dr Diff 11.7" rear disc brakes. From this point forward, the car got faster but the braking stayed the same.
Last year when I had the engine out, I decided to upgrade the brakes. I installed the Dr Diff 13" front kit and went down the rabbit hole in a failed Hydroboost swap. That resulted in a complete failure either due to a faulty HB unit or my impatience in the bleeding procedure. Regardless, the HB came back out and I tried a manual 1 1/8" master cylinder. TERRIBLE. Hard pedal with horrible stopping force. I tried a 15/16" MC. Better, but still not nearly good enough to be content with it.
I put the A body booster back in along with a vacuum pump and tank and that is where I am today.
It stops good enough but I'm still wondering why it won't skid. To me, it seems like if it can't skid, then the system isn't at it's potential.
I bought a brake caliper gauge....

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I only measured the rear and it registered 1200 psi.
I'm open to any and all suggestions except swapping drums back on the rear. There is something wrong with the system and I just am not seeing it.
Can you skid on dry pavement?

The hydroboost is what you need. You should be able to achieve 1500 psi on the fronts. I don't know what unit you used, but all my experience is with Hydratech and my results have been outrageous. Secondly, I don't know what type of linings you have but anything generic is pure junk. We have carbon kevlar pads and shoes for everything. I highly doubt you'll be able to make that system work the way you want without a HB.

 

[Kern Dog]

Thanks Peter.
Have you read the last page of this thread?

I agree that what made the difference is pressure. It feels like I am where I needed to be all along.
Before last Summer when I rebuilt the engine, I had the popular Cordoba 11.75 front discs with the huge 2.75 single piston calipers. I also had the single diaphragm 73-76 front disc A body booster. The car stopped good but wasn’t impressive.
When I changed to the Dr Diff Cobra 13” brakes, the braking didn’t improve as I had hoped.
My experience with Hydroboost was bad because I simply couldn’t get much help in getting the system to work. I agree that if it did work, it would have been great.
My deductions here are that all along, the braking would have been better had I originally had a dual diaphragm brake booster that is OEM for the 66-70 front disc/rear drum cars.
Now it feels what I’d consider “normal” for a new car.
Think about how you get into your late model car and you don’t even worry about the brakes. They just work and you have confidence in them.
That is where this feels. Light braking gently slows me down but more and more foot on the pedal and the braking force follows. I’ve had the A body booster in the car for over 20 years and didn’t know how much better I could have had it.

 

[1STMP]

Are the RPVs in these blue things?

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You wrote "Proportioning valves installed"....Did you mean Residual Pressure Valve "RPVs ?
From what I've read, NO new master cylinders have them anymore. Am I wrong on that?

"adjustable proportioning
valve" just below the
booster. (silver knob).
RCV is residual check valve.

 

[RemCharger]

I had to do the Kern Dog " make it right" conversion for a friend. Now it stops. A bit low on vacuum from the engine, but we may work on that.
The chinese/GM garbage was hurting my eyes, but that's an old memory now

 

[Don Frelier]

A black brake booster is beautiful.

 

[Kern Dog]

That red 64 Mercury that was on a video feature from The YouTube channel “Autopia” really inspired me to get my brakes working right.
That video had a guy driving that cared more about being featured in a video than the safety of himself and others. Even so, the engine was far more powerful than the brakes were. That isn’t smart.

 

[Don Frelier]

That red 64 Mercury that was on a video feature from The YouTube channel “Autopia” really inspired me to get my brakes working right.
That video had a guy driving that cared more about being featured in a video than the safety of himself and others. Even so, the engine was far more powerful than the brakes were. That isn’t smart.

I agree.
The brakes shouldn't have to hold back the engine power.
When his throttle stuck the first time he should have pulled over and found out the problem.
If he couldn't figure it out and correct it then shut it off and call a flat bed.