Brake issues

[TheCouzin]

So I just recently bought a 67 coronet 500. Has been swapped to a 440 from the 383. The brakes are Wilwood fronts and drums rear. They have proportion valve and line locker. The master cylinder looks to be factory. (I'll attach photo) When I go to press brakes it's high and hard and not a lot of stopping action going on. You have to pretty much stand on the brake pedal.
Things I've confirmed so far.

Removed master cylinder to confirm there is no mechanical issue like pedal travel or seized.
Confirmed there is vacuum.
There is fluid.

Is it the wrong master for this set up?

 

[TheCouzin]

Also juat confirmed master cylinder is a
1 1/8" Bore

 

[ckessel]

Get a 1 1/32" unit. Thats what I'm using on my 67 wagon. The Willwoods may need to get booted. Many people not thrilled with their lack of stopping ability. Plus replacement parts are not a parts store on the weekend find. You may be better off with the Disco Tech stuff. Also Dr Diff has that in a package plus a master sized to the needs. Another potential issue could be the booster is bad. Lots of stuff to check out. If you decide to change out your disc package, wheel size comes into play. If you have 14's, you can't swing more than a 10.75" rotor. 15's allow you to run 11.75. All of the below items are easily had at parts stores. Something to think about since you're in the Great White North.
https://www.google.com/url?sa=t&sou...AQFnoECBkQAQ&usg=AOvVaw3Nolx5IULUtvzcdnOVBFrv
doctordiff.com

 

[Sixpactogo]

I suspect you are using a drum brake booster. Probably your problem. Disc brakes require almost double the boost. That is the reason they created the dual diaphragm booster. The master should be ok for you IMO. I went thru, the same issue when I installed front discs on my 68 Charger. Hard pedal and not enough stopping power. One like this should heal your brake issue. It fixed mine.
Note: You will have to enlarge the opening in your firewall to accept the rubber boot on these since they are a bit larger than the factory boots. A couple minutes with a dremel tool or file will fix that problem.
https://www.ebay.com/itm/256590935169?_

 

[TheCouzin]

Get a 1 1/32" unit. Thats what I'm using on my 67 wagon. The Willwoods may need to get booted. Many people not thrilled with their lack of stopping ability. Plus replacement parts are not a parts store on the weekend find. You may be better off with the Disco Tech stuff. Also Dr Diff has that in a package plus a master sized to the needs. Another potential issue could be the booster is bad. Lots of stuff to check out. If you decide to change out your disc package, wheel size comes into play. If you have 14's, you can't swing more than a 10.75" rotor. 15's allow you to run 11.75. All of the below items are easily had at parts stores. Something to think about since you're in the Great White North.
https://www.google.com/url?sa=t&sou...AQFnoECBkQAQ&usg=AOvVaw3Nolx5IULUtvzcdnOVBFrv
doctordiff.com

Thanks for the reply. Greatly appreciated. I will take a look more into it with what you sent.

 

[TheCouzin]

I suspect you are using a drum brake booster. Probably your problem. Disc brakes require almost double the boost. That is the reason they created the dual diaphragm booster. The master should be ok for you IMO. I went thru, the same issue when I installed front discs on my 68 Charger. Hard pedal and not enough stopping power. One like this should heal your brake issue. It fixed mine.
Note: You will have to enlarge the opening in your firewall to accept the rubber boot on these since they are a bit larger than the factory boots. A couple minutes with a dremel tool or file will fix that problem.
https://www.ebay.com/itm/256590935169?_

Just adjust the proportioning valve since its still drum rear I suspect if this booster works?

 

[dan juhasz]

Are you using a vacuum pump for the booster? Have you checked to see the pump can supply adequate vacuum, how many inches of vacuum can it pull? Have you checked to see if the booster holds vacuum when applied, or does it just leak out. Either inadequate vacuum supply or the booster is dead.

 

[Don Frelier]

I have the stock booster on my Plymouth (at least I'm pretty sure it is) with a disc brake conversion on the front.
Similar master cylinder.
Brake pedal function is fine.

 

[Sixpactogo]

Just adjust the proportioning valve since its still drum rear I suspect if this booster works?

Not sure what you are suggesting here?? A proportioning valve is already adjusted to tone down the pressure to the rear drums. If you have an adjustable prop valve, you can fine tune the pressure to the rear. It does nothing for your front brakes. If you don't have enough boost for the front discs, you are pissing in the wind so to speak. Drum brakes require about 400 PSI to activate them. Disc brakes require about 800 PSI. Just because your booster is working, does not mean it provides the needed boost. Here is the formula for an 8" booster: 4" X 4" X 3.14=50.24 square inches X 8 = 401.9 pounds of force. That is enough for drum brakes. Since Disc brakes require 800 lbs, You need a dual diaphragm 8" booster giving you 803.8 lbs.
If you had an 10" single diaphragm booster the formula would be: 5 X 5 X 3.14=78.5 X 10=785 pounds of force. (better but still a bit short)
Most cars don't have enough room for a 10 or 11" booster. That is why they designed a dual diaphragm booster.

 

[TheCouzin]

Not sure what you are suggesting here?? A proportioning valve is already adjusted to tone down the pressure to the rear drums. If you have an adjustable prop valve, you can fine tune the pressure to the rear. It does nothing for your front brakes. If you don't have enough boost for the front discs, you are pissing in the wind so to speak. Drum brakes require about 400 PSI to activate them. Disc brakes require about 800 PSI. Just because your booster is working, does not mean it provides the needed boost. Here is the formula for an 8" booster: 4" X 4" X 3.14=50.24 square inches X 8 = 401.9 pounds of force. That is enough for drum brakes. Since Disc brakes require 800 lbs, You need a dual diaphragm 8" booster giving you 803.8 lbs.
If you had an 10" single diaphragm booster the formula would be: 5 X 5 X 3.14=78.5 X 10=785 pounds of force. (better but still a bit short)
Most cars don't have enough room for a 10 or 11" booster. That is why they designed a dual diaphragm booster.

I do suspect the booster is single diaphragm so that will probably be what I try to find. Looks like in a previous post above that someone provided a link to a dual diaphragm with some minor modifications to firewall. Thank you for your input!

 

[TheCouzin]

I have the stock booster on my Plymouth (at least I'm pretty sure it is) with a disc brake conversion on the front.
Similar master cylinder.
Brake pedal function is fine.
View attachment 1875361

I wonder what diameter your master cylinder is. 1 1/8"?

 

[TheCouzin]

Are you using a vacuum pump for the booster? Have you checked to see the pump can supply adequate vacuum, how many inches of vacuum can it pull? Have you checked to see if the booster holds vacuum when applied, or does it just leak out. Either inadequate vacuum supply or the booster is dead.

Not a vacuum pump. But it is pulling vacuum at the pump via hose. I am just getting a pump to test the booster to see if it holds vacuum. Will report back when I get to the bottom of this.

 

[RemCharger]

It appears you have the factory 10" power drum booster, with a disc brake master.
Someone has already been working on the problem with the extra vacuum assist stuff. Then sold the car lol

 

[TheCouzin]

It appears you have the factory 10" power drum booster, with a disc brake master.
Someone has already been working on the problem with the extra vacuum assist stuff. Then sold the car lol

Could you elaborate on that? 10" is single diaphragm? I need the 8" dual diaphragm?

 

[RemCharger]

Could you elaborate on that? 10" is single diaphragm? I need the 8" dual diaphragm?

I meant it's original for a 10" drum brake system. I believe it's a kelsey Hayes? Not sure on the technical size of it..

But yes, the factory supplied the bendix dual 8" for disc brake systems because it develops much more assist.

 

[TheCouzin]

I meant it's original for a 10" drum brake system. I believe it's a kelsey Hayes? Not sure on the technical size of it..

But yes, the factory supplied the bendix dual 8" for disc brake systems because it develops much more assist.

Thanks

 

[ckessel]

And the Doc has the right booster plus his actually work.

 

[Ron H]

Lots of great recommendations. Before you replace parts, check some basics in case you haven’t. As mentioned, do vacuum checks on the booster using gauge/vac pump. Check booster valve and lines to detect any vac leaks, collapsed (collapsing hoses/lines). Is the motor beefed up any, hotter cam? Check your idle and cruising vacuum. Power brakes typically require a minimum of 17”. Take off front wheels, with a helper pressing the brakes assure the pistons are retracting and no binding of the pads on the guides. Have you bled the system and the master? Two separate chores. If careful protecting all around from fluid purge, the MC can be bled in the car, though on the bench is better. One other tidbit is measuring the rod gap tolerance btw the booster and MC.

This is just my O and may not be your hassle, but I’d want to rule out these elementary things before getting more invasive...

 

[TheCouzin]

Lots of great recommendations. Before you replace parts, check some basics in case you haven’t. As mentioned, do vacuum checks on the booster using gauge/vac pump. Check booster valve and lines to detect any vac leaks, collapsed (collapsing hoses/lines). Is the motor beefed up any, hotter cam? Check your idle and cruising vacuum. Power brakes typically require a minimum of 17”. Take off front wheels, with a helper pressing the brakes assure the pistons are retracting and no binding of the pads on the guides. Have you bled the system and the master? Two separate chores. If careful protecting all around from fluid purge, the MC can be bled in the car, though on the bench is better. One other tidbit is measuring the rod gap tolerance btw the booster and MC.

This is just my O and may not be your hassle, but I’d want to rule out these elementary things before getting more invasive...

Thanks for the input. Yea its a bit of a built 440. Has 542hp and 574tq. I will check and recheck the vacuum numbers at various points.

 

[Geoff 2]

Ok. A 'built' 540hp 440 is going to need some 'cam'......which gives less engine vacuum to operate the booster. A quick test to see if the booster is working: press on the brake pedal 10 times to remove all vac from the booster. With your foot on the brake pedal, start the engine. If booster is ok, pedal should move slightly. I would get a 1" m/c irrespective of what else you do; it will give you 21% more output pressure.