Brake booster HK Vs smaller unit??

[Ed]

Hi guys,

I currently have a BIG HK brake booster of the V8 model variety I believe.

the car stops beautifully using HQ discs and drums.

now.. if i change out the BIG booster for something smaller.. very much smaller say off a Mazda, but still using the same 1 inch master cylinder.

Should I expect
1) greater pedal effort required?
2) reduced brake efficiency?

also has an HK master cylinder got the same mount as an HQ master?  ie are the 2 bolt holes the same spacing apart?

thanks for your help.

Cheers

Ed

[cruiser056]

Ed,
      On my FE sedan I am running a Gemini brake booster with a, dare I say, ford master cylinder.  I am running HZ brakes on the front and VN disc brake diff in the rear.  When I did the road test for the Engineers compliance he said the results were as good as a new Commodore or Falcon.  I have no problems braking and the booster setup fits very well in the engine bay.

Michael

[FC427]

Ed I have found that if the donor car is close in weight to the car it is being fitted to, and the pedal ratio is near the same you will generally have no problems. I have set up a drag car with a 1" master cylinder HZ calipers and ford rear drum brakes NO booster but with a pedal ratio of 11 to 1 and it stops good .Get Boosted have a 7" double diaphragm booster that would probably suit your car.

[mcl1959]

I think you will have slightly higher pedal effort as the small booster wont deliver the punch that the double diapghram HK unit will, but brake efficiency will remain the same.
The only negative will be that you may run out of vaccuum after several quick stops in a row.  Especially if your car is a bit cammy and doesn't produce a lot of vaccuum in the first place.

Ken

[Ed]

Hi Michael,
the gem booster was going to be swapped in, but ive located a substitute which will work with what I already have pre-existing.. hoping it wont be much trouble.
I do like the gem and ford booster arrangement tho.

Hi FC427,
thanks for the tip, currently I am boosted but looking for something more aesthetically pleasing... think ive found it.  sounds like you've had some nice cars!

Hi Ken,
thanks for the brake advice especially about the cam, ive deliberately gone for a short duration and moderate lift cam to avoid all those probs.
hoping I wont need to do too many quick stops in succession, but will make sure it's safe.

thanks all

Cheers

Ed

[FC427]

Ed I have found some info on the Gemini boosters and they are only rated at 630 psi as are the early Toyota boosters . The 7" dual diaphragm are rated at 1000 psi and the 8" dual diaphragm are rated at 1100 psi are imports from the USA but there is a few agents out here . Brake performance should not be compromised just for ascetics. JUST STOPING THAT BIT BETTER MAY BE THE ONLY THING THAT SAVES YOUR LIFE ONE DAY  Mark

[Ed]

Hey Mark,

that's very good advice.

just a question though, and with respect for your previous comment.. surely the hundreds of rods out there using the gemini set up must prove its Ok for the street???

Cheers

Ed

[mcl1959]

Not really. Every car is different and pedal ratio plays a big part. Most rods run under floor boosters where the pedal and pedal box is custom made to suit the application. You will be using original pedal box and pedal and making this work on a booster originally intended for a lighter car. Most hot rods are lighter in weight than an early Holden so do not require as much braking force as well
The other issue is that a lot of hot rods were built pre the new rules so whilst they may be legal, it is possible that they do not meet current rules.
Generally a Gemini or other small car booster will be OK on a brake performance test for the first stop but it will fade after the mandatory repeated stop test.  Just how much it will fade depends on the condition of the booster, the condition of the master cylinder, the size of the caliper pistons and rotors, the condition of the pads and rotors and finally on the choice of the pads - YES the choice of pads does matter!!!
Cheap pads take more effort to pull up a car than quality pads, race pads dont work well cold either.

So the final comment - It may well be that the Gemini booster is OK on the conversion, but it will depend on the day when it is tested.
I can tell you that at Hoppers Stoppers we wouldn't use a Gemini booster on an early Holden we would use the double diaphragm 7 inch booster as described by FC427

Ken

[Ed]

Thanks Ken,

Exactly the type of info I was after

Cheers

Ed

[GOA350]

Hi Ed,
        I have the smaller type booster from a Mazda and it works beautifully. Even doing successive stops with a solid Crane lumpy cam I don't get fade. This was proven during the engineers report and at the Jindabyne Nationals when I took a few people for a ride, and they were impressed with the braking performance.
 I hope this information helps, if you want to come to my place and take it for a drive you can see for yourself,
                Cheers Scott

[FC427]

Ed just like to let all in on the fact that we are talking a Cast iron V8 FC and not a car that weighs around 1100kg's just a bit more than a Gemini. What did your cars weigh bridge ticket read?  . Mark

[Ed]

Hi Scott,

thanks for the info, and the offer to take it for a spin  
I have full confidence it works for you.

Mark,
the wagon weighed in at 1340kg on the weighbridge with a 3/4 tank of fuel.
it maybe weighs slightly less now that Ive pulled the TH400 out and replaced it with a TH700.

yes the wagon does get about with a big ol' V8, having owned a hot little Gemini the cars are definitely worlds apart. Scott's car provides a useful real world example as it's running a 350 and very well set up.

The wagon was never repeatedly tested using the existing set up but it was put up on a "brake table" where it tests the effectiveness of each brake on each corner independently.. the inspector rolled it on and off a few times.  Each brake had to operate within a certain limit of precision and bias from front to rear.

A neat little setup I had never seen before.

as the car is, the brake pedal has always had a soft feel and the brakes are effective without fade.  One thing I have never been able to “fix” is that soft brake pedal. I am certain there is no air in the system, and all brake system components were fitted new

I may swap the boosters over and then get it tested, if it proves to be dangerous then it will be tossed.

The amount of constructive info you guys have offered up has been great.

Cheers

Ed

[FC427]

Ed have you played with the reaction disc behind the master cylinder push rod in the booster, there are all different  grades and are colour coded for identification [ I have the grades some where ,had a look but could not find them at the moment ] and they change the feel of the pedal Mark

[Ed]

Hey mark,

Nope, never even knew about such a thing!
I had the booster rebuilt by a pro, similarly the master cylinder.
I just got them back and assembled.  may be a good time to play as the booster is out.

Cheers

Ed

[Ed]

GUys,

After much thinking and comparison of the 2 boosters I have decided not to change to the smaller unit.

it looks fantastic and everything and by all user accounts works very well, but the rationale behind changing it just doesn't add up.

considering I do tow trailers it's probably best left alone (ie working well).

if it aint broke.....

Cheers

Ed