Stevec

The previous owner of my 68 442 converted the front drums to disk and the pedal has always been hard and braking not so good.

Now that I have a 500+ hp 468 under the hood nows the time to get the brakes in order.

The conversion kit is from ssbc so I contacted ssbc to see what size bore was in the master cylinder that was in the conversion kit,they said 1 1/8. I researched online bore sizes and read that for 4 wheel disk I should use 1" bore.

This is the list of parts I used so far
Ssbc 4 piston calipers
Ssbc drilled and slotted rotors
7" booster,had to use a smaller booster to make room for the new engine
Vacume pump
Master cylinder 1"bore
New proportion valve
All new brake lines

Still hard brake pedal that's not very effective
Was thinking of getting another booster,master cylinder and proportioning valve that are sold as a complete kit.

I'm stumped!

Thanks

joe_padavano

I'm certainly not stumped.

The assist provided by the brake booster is a function of the area of the diaphragm in the booster. Your 7" booster has a 38 sq in diaphragm. The stock 11" booster has a 95 sq in diaphragm. The smaller booster only provides about ONE THIRD as much assist as the stock booster at the same vacuum levels. Since a smaller booster is usually needed because of a radical engine, the vacuum will be even lower. I have no idea how good the vacuum pump is, but even if it's perfect, you only get 14 psi differential across the diaphragm.

Your options are to either figure out a way to run a larger booster, run a hydroboost, or trash the booster and run manual brakes. FYI, my 70 W-30 with factory manual disk brakes stops fine and does not require excessive pedal force. The manual brakes use a different pedal ratio, so pedal force is reasonable even without power assist but pedal travel is greater. A member of our club had exactly this problem on his 68 442 with a similar engine and 7" booster. After spending a lot of money on different master cylinders, vacuum pumps, etc, I convinced him to run manual brakes. Now he's happy with the braking.

Stevec

Thanks Joe, I have thought about goin to manual brakes. I think the booster is bad also,even with the vacume to booster hose disconnected there is no change.

I even tried starting the car with my foot on the pedal and there is no pedal movement upon start up.

Again thanks for the reply

rustyroger

It sounds like your booster isn't working, even if it wasn't much help you should feel a little pedal travel if you have your foot on the brake pedal at startup.


In the '60s to get round overcrowded engine bays in out small cars in the UK several manufacturers used remote boosters, they could be mounted out of the way of the engine, in theory I suppose you could mount it on the roof if you wanted to. Several people preparing Mini's for rallying put them in the trunk.


Roger.

Stevec

Thanks Rodger

jaunty75

I love British English! Or what you probably call simply English. Over here, we'd say "to get Around overcrowded engine bays."

joe_padavano

The device you're talking about is a Hydrovac (not to be confused with a Hydroboost, which is a hydraulic booster that uses power steering pump pressure). The Hydrovac has been around since the 1940s and was quite common on trucks. The problem is that the Hydrovac only works with a single brake circuit, not a dual master cylinder.

As the diagram below shows, the primary master cylinder operates a boost valve that controls vacuum in the large piston cannister. This acts like the diaphragm in a conventional vacuum booster and pushes on the secondary master cylinder, which in turn activates the brakes. Note that the primary master cylinder also has a direct hydraulic connection to the secondary master cylinder as a fail safe.

rustyroger

What you have shown is different to the remote servos I'm thinking of Joe.
I owned a Humber Sceptre a few years ago with what I was referring to, I'll see if it has a clear description and diagram.
Just occurred to me, when My Humber was built Chrysler had a controlling interest in Rootes Group who built it. Made me a Mopar owner by marriage perhaps?.


Roger.

joe_padavano

I guess I'd be interested to see it. If you search around on ebay.uk, you find a lot of remote servos for various RH drive cars, like Cortinas, Austins, etc. The shape may be different (diaphragm vs. piston for the vacuum cannister, for example), but they all essentially function the same way. The primary M/C provides hydraulic pressure that actuates a control valve at the remote servo that modulates vacuum to the piston/diaphragm and thus operates a secondary M/C that is directly connected to the brakes.

joe_padavano

OK, here's the engine compartment of a 1965 Humber Sceptre. Note the remote servo (booster) and secondary M/C in the lower RH corner of the photo.



And speaking of a face that only a mother could love... here's a 64 model.



By the way, a trivia point on the Chrysler/Rootes connection. Sunbeam was owned by Rootes Group, along with the Sunbeam Alpine. Of course, this car begat the SBF-powered Sunbeam Tiger. Chrysler was not thrilled with acquiring production of a car powered by a Ford motor, and the small block Chrysler did not fit easily, so this is why the Tiger was killed.

oldcutlass

I think the most iconic American V8 powered car from the late 60's through about mid 70 was the Jenson Interceptor. BB Chrysler in a small car, great lines for a British car, and plenty of power. Rumor has it they are coming out with a 2014 model that's Viper V10 powered.

KDV

I don't know how much vacuum your pump creates, but I had the same problem with the 7" booster and drums all the way around.

I switched to an 11" booster and still had to run a vacuum canister to run the assist as my engine was only producing about 14" of vacuum.

Works great now!

IMG_20130712_110802_964_zpsee72c716.jpg

Stevec

I ordered a 8" dual diaphragm booster, supposed to provide more assist than a 11" single diaphragm.

We shall soon see!

joe_padavano

Atmospheric pressure is only 14.7 psi at sea level. You can't physically do any better than that, no matter what pump you use. This is why brake power assist is dependent on booster diaphragm area.

Also, an 8" dual diaphragm booster has about the same diaphragm area as an 11" single diaphragm booster, so brake assist will be about equivalent with either for a given vacuum pressure.

Stevec

Installed the dual diaphram 8" booster......a lot better! I am going to install disk brakes in the rear this spring and that should be it for the brakes.