Brake experiment

[Big_John]

So.... I did a little searching, figuring others must have this problem too.

I didn't find much, but hit on an old thread in a vintage Mustang forum. Someone suggested a vacuum regulator or a vacuum limiter. The suggestion was to get it to about 10"hg and there would be less vacuum and therefore less boost and higher pedal pressure.

I didn't go any farther with this and the links that were in the thread were 10 years old and dead.

Might be something to consider.

 

[rkrochen]

So.... I did a little searching, figuring others must have this problem too.

I didn't find much, but hit on an old thread in a vintage Mustang forum. Someone suggested a vacuum regulator or a vacuum limiter. The suggestion was to get it to about 10"hg and there would be less vacuum and therefore less boost and higher pedal pressure.

I didn't go any farther with this and the links that were in the thread were 10 years old and dead.

Might be something to consider.

Another good idea and easy one to try.

 

[darth_linux]

You may be right I just need to think about it. Anyone else wish to jump in please do so.

deleted due to inaccuracy. it was worth a try . . .

 

[Imperial dude]

I found that replacing the brake booster resulted in a less touchy pedal. If your wife will drive the car, you may want to think about an upgrade to disc brakes.

My 68 Monaco had that, I used to describe it, you hit the brakes like there was an egg on the pedal, the booster went bad and I replaced it, no more problem
Worth a shot

 

[Trace 300 Hurst]

just like putting your thumb over the end of a yard hose increases the pressure of the water coming out, and taking your thumb off decreases the pressure, a smaller piston bore would have the same effect - multiply the pressure without increasing the volume. A larger piston bore would reduce the pressure, while still flowing the same volume. My $.02

No, sorry, wrong. Putting your thumb on the hose increases the VELOCITY, not the pressure of the water coming out. At that point the pressure of the water is zero. The increase in velocity from that thumb increases the distance that the water sprays. Don't confuse pressure with velocity. Or volume, for that matter. They are related, but not collaborated.

This "hose thing", which is an open system, has nothing to do with hydraulic brake system pressure, which is a closed system.

My post above, #36, very much explains the issue.

 

[rags]

Smaller would give more travel, but it will increase pressure.

agree. think of the pistons in a floor jack

Has anyone noticed changes with their dual cylinder upgrade.

on my '65 drum/drum car, the brakes would stick bad on the first couple of applications each day then work normally. converted to dual master for '69 drum/drum car. made no difference.

 

[rkrochen]

agree. think of the pistons in a floor jack

on my '65 drum/drum car, the brakes would stick bad on the first couple of applications each day then work normally. converted to dual master for '69 drum/drum car. made no difference.

 

[rkrochen]

I have thought thru this and yes the smaller piston creates more pressure but requires a longer stroke whereas the larger
Piston creates less force but moves more fluid.

 

[Triple Pickle]

I have thought thru this and yes the smaller piston creates more pressure but requires a longer stroke whereas the larger
Piston creates less force but moves more fluid.

Exactly. But which one creates “less touchy” brakes? On a manual system it’s easy to imagine the effects: smaller=more travel, less effort. Bigger= less travel, more effort.
Same effect on power brakes but with the boosters assistance going bigger could make it more “touchy” couldn’t it? Not saying it would but it’s something to think about.
If there’s an over abundance of power more resistance won’t slow it down.

 

[rkrochen]

Exactly. But which one creates “less touchy” brakes? On a manual system it’s easy to imagine the effects: smaller=more travel, less effort. Bigger= less travel, more effort.
Same effect on power brakes but with the boosters assistance going bigger could make it more “touchy” couldn’t it? Not saying it would but it’s something to think about.
If there’s an over abundance of power more resistance won’t slow it down.

Exactly. It’s a bit of a shot in the dark. I think at this time I will find a dual cylinder that fits and do the conversion. From there I will determine the next step if needed.

 

[CBODY67]

IF the booster is the issue, which it could be, might a call to @BoosterDewey be in order? Possibly they might have something internal which could be altered to provide "better pedal feel" for less sensitivity?

When going from the '66 Newport to my '67 Newport (dual master cyl), I noticed no real difference. OR going from my '67 Newport to the '70 Monaco to our '72 Newport (power disc drum) cars. But THEN I was also used to driving them with NO differences in brake pedal feel, other than the '72 Newport with the front disc brakes had a slightly spongier feel compared to drum brakes. Which I have noticed on all of the power disc brake cars I have owned/driven (even non-Chryslers).

Our old service station operative did put the end of a tire valve stem into the pressure line on his '65 Fury IIIs (318 cars) to decrease the power steering boost. Doing similar with a power brake vacuum supply would not work the same way as the vac booster accumulates boost until needed. It's that "until needed" aspect that would defeat the purpose, to me.

I kind of like Sir Rick's idea as it would be better than installing a f/r aftermarket (although Mopar Perf and GM Perf Parts sell them for on-track purposes) proportioning valve. Easier to do and non-invasive to the piping system.

I would suggest doing the dual master cyl upgrade and see how things might progress from that. For general information, might also research the FSMs of various years for master cyl bore diameters and how they might interact with wheel cyl bore diameters.

On the vehicle assy lines, things need to be as common as possible to prevent issues of wrong parts, right car. Which would leave brake lining widths to be the main changeable items, or different master cyls.

Just some observations,
CBODY67

 

[thethee]

So.... I did a little searching, figuring others must have this problem too.

I didn't find much, but hit on an old thread in a vintage Mustang forum. Someone suggested a vacuum regulator or a vacuum limiter. The suggestion was to get it to about 10"hg and there would be less vacuum and therefore less boost and higher pedal pressure.

I didn't go any farther with this and the links that were in the thread were 10 years old and dead.

Might be something to consider.

This should be quite possible now that I think about it. Every time I read threads on cam selection there would be someone pointing out if they had power brakes or not since not enough vacuum would render them useless. Mounting some vacuum regulator in the booster hose and you're good right? Just takes some trial and error in dialing in the regulator.

 

[rkrochen]

This should be quite possible now that I think about it. Every time I read threads on cam selection there would be someone pointing out if they had power brakes or not since not enough vacuum would render them useless. Mounting some vacuum regulator in the booster hose and you're good right? Just takes some trial and error in dialing in the regulator.

I quickly looked into sourcing a vacuum limiting valve without any luck. Not sure one is made. The purpose would be to limit to amount of vacuum in the booster. As mentioned I. The previous post vacuum will continue to accumulate as brakes are not utilized all the time.

 

[CBODY67]

To limit total vac to the booster, you might take an AIR system check valve, place it near the vac source at the intake manifold vacuum tee, backwards (?), then get an item from Grainger the Turbo 2.2L racers used to bleed-off vacuum to the wastegate control, for higher total boost pressures, and put that near the booster. That way, the variable vac bleed item should not affect intake manifold vac and such, yet still provide consistent lower vac to the booster (with more effort required to push the brake pedal). But that call to BoosterDewey would be best.

 

[rkrochen]

To limit total vac to the booster, you might take an AIR system check valve, place it near the vac source at the intake manifold vacuum tee, backwards (?), then get an item from Grainger the Turbo 2.2L racers used to bleed-off vacuum to the wastegate control, for higher total boost pressures, and put that near the booster. That way, the variable vac bleed item should not affect intake manifold vac and such, yet still provide consistent lower vac to the booster (with more effort required to push the brake pedal). But that call to BoosterDewey would be best.

I am still in Mexico and will call BoosterDewey when I get back.

 

[Big_John]

I quickly looked into sourcing a vacuum limiting valve without any luck. Not sure one is made. The purpose would be to limit to amount of vacuum in the booster. As mentioned I. The previous post vacuum will continue to accumulate as brakes are not utilized all the time.

Amazon product ASIN B0BMZGDBFP

 

[Big_John]

More money, but easier to adapt to the vacuum hose.

Amazon product ASIN B003TPF85A

 

[rkrochen]

More money, but easier to adapt to the vacuum hose.

Amazon product ASIN B003TPF85A

This one looks like it could do the job. I will definitely consider trying this. Thanks for the help.

 

[rkrochen]

I found an interesting article about brake hydraulics which may be of interest to some. Master Cylinder Math - JOES Racing Products

 

[fury fan]

If a new booster doesn't solve the problem like it has for some folks -
You could consider putting a manual-brake pedal on with the vacuum booster (and evaluate as prudent).

I did this to my 65 Fury years ago by mistake, before I knew that the pedals differed between MB and PB. (by years ago, I mean about 30 of them)
I converted the car to disc brakes, installed the disk-brake booster (all parts from a 71 C-body), had to modify the '65 pedal bracketry a little for the nose of the booster, but otherwise everything fell together like it belonged. Modifying the brake pedal seemed par-for-the-course as the parts were 6 years apart, Fusey vs Slab.

When finished, the brake pedal always felt soft. (I'll spare all the diagnostic checks and all the 'experts' who couldn't solve the problem)
But the car stopped really well, better than our then-new 91 Buick, and on par with our 91 Camaro.
The pedal has the longer travel of the manual brakes and reduced effort of power brakes.

Might it give the desired result here? You would need to decide for yourself, though.
It definitely would need review for whether it over-strokes the booster, but that might be a simple geometry calculation between the 2 pedal setups. (i.e. do they have near-equal pushrod travel?)
My hunch is that it might not matter, as the MC piston travel (and therefore pedal travel) depends on when the hydraulic pressure causes the brake parts to bottom out at the wheels.

As I think about it, I suppose one might be able to over-pressurize the hydraulics due to extra mechanical leverage with the vacuum boost, but I suspect that SAE engineering principles dictated for safety factors in the seals, hose construction, etc, to be far more robust than the hydraulic pressure in the system. Or does the booster simply add a constant amount of additional force, independent of the force from the brake pedal?

And regardless - in normal driving, you won't be pushing the pedal anywhere near as hard as in a factory MB or PB setup, and obviously you'll adjust your leg to the required braking.

So it would boil down to - if you pushed the brake pedal as hard as physically possible, harder than you'd ever need to, like you were trying to break something - what happens?
Does somethign break? Hose burst? Wheel cylinder cups blow out? Does the MC piston merely bottom out in the bore? Or nothing unusual happens?

One advantage in this setup over mine - it is using the drum-brake booster, which gives less boost than the diskbrake booster in mine, so less force available to break something.