1 (edited by zanzabar88 2017-09-06 05:22 PM)

Topic: school me on master cylinder design

I'm not sure I understand this properly, so looking for someone to explain it for me. I know next to nothing about hydraulic physics.

I'm altering a needlessly complex brake system by doing the following:
- removing the front brake booster that was originally fed from one port on the master cylinder, the line ran back to the rear of the car to where the booster was located, then all the way back up to the front and then to a splitter that went to one caliper on each side.
- replaced original master cylinder (rusty one on the right) with one that has two separate ports for the front, and will run a separate line from here directly to each caliper.
- The rear brakes will remain stock (no booster) with a single line running from one port on the MC to the rear splitter.
- The result will be much simpler, but time will tell how well it stops the car!


My question is, is it reasonable to assume that two lines directly from the MC -one to each front caliper- will provide more clamping force at the calipers than a single line to a splitter that then feeds each caliper? Or am I misunderstanding the physics here?

Captain of McDads/A Fart Racing - new team for 2017 - 1977 Lancia Scorpion
Captain of 42 Hours of MeLons (2013-14) - Vattenmelon Vagn 1984 Volvo 240, B-Class Winner: Arse-Freeze 2014

Re: school me on master cylinder design

There are two issues at play here:
- As long as there is no gas/vapor in the brake lines, and only fluid, any pressure applied to the system is equal throughout the system.
    That discounts any frictional losses, and only after all movement of various pistons stops. Clamping force is the same regardless.

- The other issue is a single supply split to two caliper vs. two individual lines - The single line can only move a certain amount of fluid, 1/2 to
   each caliper. That may mean a longer pedal travel depending on the MC design. But looking at the pic, it seems that the two ports feed from
   the same piston, so all you've really done is make your single line split into two, very short and fat, and your wheel lines really long.

Should work well, though you may miss the booster.

Capt. Delinquent Racing
The One & Only Taurus V8 SHO #31(now moved on to another OG Delinquent)
'17 Vodden the Hell - (No) Hope for the Future Award, '08 AMP Survivor, '08 ARSE-FREEZE-APALOOZA Mega-Cheater

Re: school me on master cylinder design

Cripes what a nightmare.

Hmmmm. the master on the "Servo Unit" may be a diffrent displacement than the "driving" master that directs the Servo Unit.

Be certain you can run both front brakes with the displacement of the "Driving" master. Check on each how much volume you move with one pump either by math or actually pumping it into a measuring container.

You'll also want to put a proportioning valve on the rear line. Looks like the Servo unit was acting as the front to rear proportion control which you now won't have.

Your life may be easier if you just switch to a dual master pedal with a balance bar really.

Then you can math out how much MC you need for front and rear

Mistake By The Lake Racing (MBTL)
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Re: school me on master cylinder design

This is the idea:

But just going to use the stock brake calipers.

So, should I get a proportioning valve on the rear line?

I haven't been able to find any specs on the cylinder volume, etc., other than that the piston diameter is 19.05mm

Captain of McDads/A Fart Racing - new team for 2017 - 1977 Lancia Scorpion
Captain of 42 Hours of MeLons (2013-14) - Vattenmelon Vagn 1984 Volvo 240, B-Class Winner: Arse-Freeze 2014

5 (edited by jrbe 2017-09-11 05:16 PM)

Re: school me on master cylinder design

I think that is the funky booster you're ditching. You should be able to drive it with the engine off, use the brakes a few times until assist goes away, and see how bad the brakes will be without it helping.
It's been a while since I've done anything with one of those, but I think it about doubles brake pressure. Just a foot feel, nothing scientific there.
It's really hard to say how balanced the brakes were, or how bad they will be without front assist. An adjuster on the rear will only make sense if you have enough brake to stop and if the rear is over biased.
There's the potential of changing the lever ratio of the master cylinder to pedal. Though if I remember right, under the steering column in those is chaos. Not sure you have room to do much under there without big changes.
I'd hang onto the booster just in case you realize it was a bad idea removing it.
There's also the challenge of that master being able to make a lot of pressure if you do up the pedal ratio. I was told they used leather as the seals..no idea if that's true. It might not be up to the new task.

-Killer B's (as in rally) '84 4000Q 4.2V8. Audis never win?

Re: school me on master cylinder design

I'll start by saying I know nothing about this car, but I won't let that stop me from give you advice.  Seems brake balance is achieved by increasing front line pressure via the booster.  If you eliminate the booster and re-balance the system with conventional proportioning valve, then the rear pressure will be decreased.  Now the whole system will be running at a lower pressure than it was designed for and I would guess you would have poor braking.
As mentioned you could move the pivot point on the pedal to increase pressure from the master.
Or is there anyway you can mount a newer style vacuum assisted booster and master cylinder up front.  Then hard line a vacuum hose from the engine to the booster?

If it doesn't have 2 doors, 3 pedals, and 5 lug nuts per wheel - It isn't a real race car

Re: school me on master cylinder design

So, we raced at Buttonwillow (eventually, on Sunday) and the car was ok. Brakes weren't too bad, though it was so underpowered and slow that we really didn't fully test them. The pedal travel was quite long, you really needed to pound on it to get the necessary stopping force, but on the other hand it left a lot of range for modulation. I never was able to lock up any of the four tires that I know of, so overall pressure is perhaps less than ideal, but at least the balance seems pretty darn ok.  In summary, the system worked, but still could be improved. I'm not sure I want to deal with a booster, but if anyone has suggestions to increase clamping force evenly across all three lines that exit the MC then I'm all ears. I'll start exploring ways to move the pivot point on the pedal, that sounds like a good place to start.

Captain of McDads/A Fart Racing - new team for 2017 - 1977 Lancia Scorpion
Captain of 42 Hours of MeLons (2013-14) - Vattenmelon Vagn 1984 Volvo 240, B-Class Winner: Arse-Freeze 2014

Re: school me on master cylinder design

You could try residual brake pressure valves?

http://www.jegs.com/i/Allstar-Performan … bEQAvD_BwE

Apocalyptic Racing - Occupy Pit Lane racing
Racing the "Toylet" Toyota Celica powered by Chevrolet Ecotec.
16x Loser with the Celica. 5 times loser in other fine machines

Re: school me on master cylinder design

You may have a problem improving your brakes with your current setup.  You mention a long pedal and you mention high effort. These should not go together. Small bore  MCs usually result in low effort and big bore high effort. Small bore usually result in longer pedal travel and big bore less. It's all about the amount of fluid moved per inch of stroke of the MC. That's why the use of a booster allows the use of a big bore MC and low effort due to the assist. 

Changing the pedal ratio may not help if the intent is to reduce travel as effort will go up.

Re: school me on master cylinder design

Seems like you need to head in the direction of a larger bore MC and more pedal leverage.

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1964 Sunbeam Imp (IOE 2013 Sears Pointless)
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Re: school me on master cylinder design

The braking effect you now have sounds inadequate.  You ought to be able to lock them up if need be.

The various ratios of M/C, caliper pistons, pedal pivots/fulcrums only change the travel/effort ratios.

All of these hydraulic issues can be kind of out of whack and still produce a good braking effect.

The mechanical side of this wasn't mentioned--pads and rotors, (the booster being a mechanical-side item that's now off the table.)  What pads are you using?

If you are committed to not using a booster, maybe you need pads with better bite to compensate for the lack of mechanical force you were used to with the booster.


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Dirty Rotten Cheating Bastards Racing

Re: school me on master cylinder design

At Arse Freeze the braking improved some. The rear calipers I discovered have a certain orientation that the pistons need to be in when bleeding or they trap some air and I believe that was what was going on at Buttonwillow. I corrected that orientation and bled them again and it was better. I think it may be possible to lock them up now but it takes a really strong leg. We're using EBC Yellowstuff pads and slotted rotors. I haven't looked at how much wear there is on the pads yet. I was overall pretty happy with the braking, I especially like how easy it is to modulate.

Captain of McDads/A Fart Racing - new team for 2017 - 1977 Lancia Scorpion
Captain of 42 Hours of MeLons (2013-14) - Vattenmelon Vagn 1984 Volvo 240, B-Class Winner: Arse-Freeze 2014