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

[fourgearsavoy]

This is something that I have considered doing....plumbing the system without a combination block of any kind.
The dual diaphragm booster is probably going to be my next move along with plumbing the lines directly.

I don't use a block of any kind on my Savoy and the brakes work perfectly. I have front discs from 73 Charger and Wilwood Dynalite on the rear. My MC is a 15/16" aluminum unit. My brakes have never locked up before though and I have really mashed them hard a few times. My tires are fairly hard M/T SR street tires.
Gus

 

[Aron Gleason]

I don't use a block of any kind on my Savoy and the brakes work perfectly. I have front discs from 73 Charger and Wilwood Dynalite on the rear. My MC is a 15/16" aluminum unit. My brakes have never locked up before though and I have really mashed them hard a few times. My tires are fairly hard M/T SR street tires.
Gus

Same here. I've got the 11 inch wilwoods up front with 10 inch drums in the back. And have done numerous panic stops without ever locking up the brakes. And I don't have a proportioning valve or distribution block in the system.

 

[Runner 68]

I haven't tested it yet but Baer 13 inch manual discs on all 4 corners. Plumbed the proportioning valve just because. Not sure if I will need it or not.

 

[1STMP]

Do you have a thread somewhere where you go into detail on what all you did and how you did it

No I didn't create a thread. The build
has taken over 20 years to build and I
picked up the parts needed for the
brakes along the way. When designing
the steering, I chose Ford Pinto
spindles as they are super strong
which set the style of front brake
components. The rear axle is a
chrysler 8.25 which has the same
axle flange bolt hole pattern used
on Jeep's D35, hence the Jeep disc
conversion kit (half the cost of a
Wilwood kit). The calipers they use
in this kit are from a Ford Explorer.
The brake lines are 1/8" I'd. (less
fluid, higher pressures). Easy to
route and bend.
Chrysler New Yorker dual diaphram
Booster (dual diaphram boosters
are recommended when running 4
wheel disc systems) which worked
well with the Dodge Diplomat brake
pedal assembly. (correct pin length
and pedal ratio). It also was a direct
bolt in behind the dash of a 80
year old Plymouth truck. Just had to
drill two additional holes in the firewall
and reconfigure the pedal arm to
gain more footwell room.
A 1.000 to 1.031 bore dia MC. (also
recommend for a 4 disc system).
2lb check valves in both front and
rear brake line circuits to prevent
fluid bleed back, which results in
less pedal travel and quicker pad
to rotor contact, and 2lbs is not
enough to cause excessive drag
on the rotor.
Note the adjustable proportioning
valve below the booster and the 2lb
check valves.
Also used in calculations was the
suspension (noted as hertz). A
frequency of 1.0 in the front and 1.3
in the rear, based on 1700 lb front
axle weight, 1400 lb rear axle weight,
two passengers and a full tank.
No cross transfer of weight
calculations are needed for straight
line braking.
Both a Granada and a Jeep outweigh
my build by close to a thousand lbs.,
so the components chosen are
adequate enough to do the job at
a much cheaper price, and are more
readily available.
Think of this as you're on a ten speed
bicycle. What happens when you
hit the front brake first? *** over
tea kettle as that momentum is
still trying to move forward. As
kids we learned to leave that front
brake alone until the rear brake
had a chance to slow some of that
momentum down a little. That same
rule of physics applies to a four
wheeled bicycle.

 

[Kern Dog]

I'm no expert on brakes by anyone's standards but I would look at the A body booster as not being up to the task. Is it a single diaphragm or dual?
I had the same symptoms on my 68 Charger using the stock drum brake booster. They don't provide enough boost for discs no matter how much vacuum you apply. I think I read somewhere that Disc brakes need about 2.5 times as much boost as drums need. For me, I would swap out that A body booster for a B body dual diaphragm unit.

The A body booster is a single diaphragm unit. The possibility of it being inadequate is is what I am thinking as well.

I tested the left front and got this reading.....

To recap, I have a vacuum pump and storage tank since the rowdy cam produces too little vacuum. I turned on the pump, let it build vacuum/pressure and had the Wife stand on the pedal.
It shot to over 1200 but dropped from that pretty fast. Within a few seconds, it was below 800 and it continued dropping. I don't think that is normal.
It seems that the pressure should remain steady.
The 1200 number is supposed to be right on target with what is expected but the way the pressure drops off so fast, it looks like I'd have to keep pressing the pedal harder and harder in a panic stop.
This is where the prospect of changing to a stock dual diaphragm booster makes sense. Maybe the greater volume in the booster would keep the pressure from dropping off so fast.
The A body single diaphragm boosters were designed for cars that weighed 3200-3500 lbs and had rear drums. I'm closer to 4000 lbs with 4 wheel discs.

 

[6872n73]

How about checking vacuum at the booster while watching the brake pressure?

 

[Kern Dog]

That could be tricky. I'd need to fab up something like an inline nipple to attach the vacuum gauge to.
I did check the vacuum pump. It draws 22" of vacuum and holds steady.

 

[1STMP]

Was there an increase in pedal effort
when the vacuum dropped off?
That first initial hit is right where it
needs to be. You may also want to
check the master cylinder to see if its'
pushing fluid past the piston O rings.

 

[Kern Dog]

The pedal doesn't drop much, certainly not in proportion to the rate that the pressure does. The Wife said the pedal does drop a little but not much.
I have noticed that the pedal travel is more than I like.
It does feel like I run out of boost as I am slowing from highway speeds....the pedal travels more and the reaction from the pedal isn't what I expect.

 

[68 Sport Satellite]

I'll say it again (although it's based on what I keep reading and I still need to try it, which is in the works for the next week) -
I believe it's backflow into the MC. If you install 2psi residual pressure valves (1 for front and 1 for rear just after MC connections), I have read that this issue you're having should go away and the pressure would remain constant. The only thing the vacuum booster is doing is making it easier to push the pedal, correct?

 

[Kern Dog]

I don't understand how an RPV would help. Someone smarter that me would have to explain it!
This sort of thing seems to happen when a hodge-podge arrangement of parts are thrown together. The front and rear brakes are stock for a 1994-2001 Mustang Cobra so it isn't as if they are junkyard parts from random vehicles.
When diagnosing problems, I have to look at the common denominator that has been in place while other things have been changed. The A body brake booster is one of those things.

 

[1STMP]

I don't understand how an RPV would help. Someone smarter that me would have to explain it!
This sort of thing seems to happen when a hodge-podge arrangement of parts are thrown together. The front and rear brakes are stock for a 1994-2001 Mustang Cobra so it isn't as if they are junkyard parts from random vehicles.
When diagnosing problems, I have to look at the common denominator that has been in place while other things have been changed. The A body brake booster is one of those things.

Hey KD,
There would be nothing detrimental
with placing a 2lb check valve in the
front circuit. All they do is hold the
fluid pressure to the front pads,
thereby, reducing the response time
of the fluid from the master cylinder
to the front pads. They do not add
any additional clamping effect, and
do not add enough force to cause
overheating.
My suspicions to your braking
concerns lay with your master cylinder.
(based on your explainations).
A 200lb drop in fluid pressure points
in that direction, unless you have a leak
somewhere. The boosters job is to
lessen the amount of foot pressure
on the pedal. Nothing more. Dual
diaphram boosters are recommended
for a 4 disc system because of higher
fluid pressures in the lines. (more
assist). Depending on your existing
brake line inside diameters, they're
part of what drives the MC bore size.
ie...my system: 1.031 MC bore with
1/8" id lines. If your lines have a larger
I'd of a 1/4" or so, a 15/16 bore in the
MC would probably work. (pushing
less fluid at higher pressures).
Hope this helps, and you get the
problems figured out.

 

[Kern Dog]

I have stock sized 3/16" brake lines. Those are another common denominator though they are the most common sized that I have seen used. They shouldn't be a problem.
How would these RPVs be installed? I know that they used to have them in 4 wheel drum master cylinders but how would you install them in a master cylinder that never had them?

 

[1STMP]

I have stock sized 3/16" brake lines. Those are another common denominator though they are the most common sized that I have seen used. They shouldn't be a problem.
How would these RPVs be installed? I know that they used to have them in 4 wheel drum master cylinders but how would you install them in a master cylinder that never had them?

Dual diaphram booster (Chrysler New
Yorker), 1.031 MC bore, 2lb RCV's in
each circuit, adjustable proportioning
valve. All up top and serviceable.
Another thing I forgot to mention
is to make sure your MC doesn't
have the proportioning valves pre-
installed. If it does, they'll have to be
removed. The MC I'm using is for a
Dodge Diplomat (1985) that had front
disc brakes.

 

[Kern Dog]

Are the RPVs in these blue things?

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?

 

[1STMP]

Are the RPVs in these blue things?

View attachment 1448779

Yes, blue is 2lb.

 

[1STMP]

Are the RPVs in these blue things?

View attachment 1448779

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?

Something else to bear in mind is the
amount of fluid you're trying to push
thru those lines.
I've got single piston calipers on all
four corners. Go up in the # of pistons,
it may or may not increase the amount
of fluid pressure required for optimal
brake performance. Part of the reason
why a 4 piston system is better than a
6 piston system.
Too many are caught up in the hype.
Minimal piston #'s with a larger diameter works just fine.
Wilwood thru the spokes sure looks
nice, though.

 

[68 Sport Satellite]

KD - yes the 2psi Residual Pressure Valves are installed in line just after the MC. Summit has the Wilwood 2psi residual valves. I have one on hand for my car (only 1 for me since mine is Front disc, Rear drum and I've confirmed my MC has a 10psi rear valve already built into the MC at the port). Order 2 steel Earl's fittings for each valve (1 for each end of each valve) as the brass ones have been known to leak due to thin wall. I'll post the Summit info for my order below. The RPV's are just one-way check valves. You already installed the vacuum can (or pump, can't recall), so going to dual diaphragm booster wouldn't do much more I wouldn't think.

Here's another photo

 

[68 Sport Satellite]

Wilwood Residual Pressure Valves 260-13706​

Part Number: WIL-260-13706
$20.72
Qty. 2
$41.44
Buy It Again

Earl's Performance Hardline Adapters 02032ERL​

Part Number: EAR-02032ERL
$7.39
Qty. 2
$14.78
Buy It Again

 

[Kern Dog]

Last week I checked the LEFT front caliper pressure.

It pegged at the number above but then dropped off as if I let off the pedal. I don't understand this. Aside from some leak, why would the pressure drop? The master cylinder isn't leaking. The lines are tight, the fittings are all dry.

Today I went for a drive. All was well for the first few miles, then the pedal travel increased. A few miles later, the travel was even more. I pulled over to find the front reservoir almost empty. Where is the fluid going?
At home, I filled it up with more DOT 5 fluid and bled the system using this: