• Registered: 2014-11-11
  • Posts: 148

Topic: Brake Feel and Booster

For some time now we've been unhappy with our brake modulation.  We've tried several race compounds, and with all of them that last we find it is too easy to accidentally lock up the brakes.  It's hard to feel where you are in the range of braking forces.  We decided this time that perhaps disconnecting the brake booster might help.  We raced saturday with it on and sunday with it disconnected.  There is no doubt that modulation was more possible.  There is also no doubt that braking became a very athletic activity.   I've been reviewing our race video footage and each of us locked up our brakes a few times on Satuday and not on Sunday, but I also noticed that our braking points moved back 1-2 cones at turns 7, 11, and the bus stop.  So we are wondering how to get less brake boost without eliminating it altogether.  Any thoughts?

My old 911 had no power brakes and was fine. I'm guessing that's because the pedal assembly had more leverage, or perhaps the master cylinder diameter was a bit smaller.  Perhaps we need a different master cylinder?

  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 600

Re: Brake Feel and Booster

How the rotor temperature? Might be overheating stuff?
switch the master cylinder for a smaller bore size and no booster.

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  • From: Tustin, CA
  • Registered: 2010-12-30
  • Posts: 3,275

Re: Brake Feel and Booster

Just a wild, out my backside shot in the dark but wondering if your car model came both with and without power brakes and somewhere along the line someone put a non-power master on a car with power brakes?

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  • From: South of Chicago
  • Registered: 2011-11-26
  • Posts: 750

Re: Brake Feel and Booster

Your options are to go with a smaller bore master but you will then have a longer pedal to make up for the loss of volume. Not a major issue but it would take some getting used to.

See if a smaller diameter booster will fit or can be adapted. Less assist may be all you really need.

Also you can try to limit the level of vacuum the booster sees by adding a restrictor or a controlled leak in the system to limit the vacuum the booster sees.

Last is to modify the brake pedal to reduce the ratio, increasing the effort and keep the booster or ditch the booster and increase the ratio to reduce effort.

I would go with a different master size or  different booster as easiest to change.

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  • Ripened Lemon
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  • From: CT
  • Registered: 2015-10-06
  • Posts: 282

Re: Brake Feel and Booster

Lots of things to look at, including the 'mechanical advantage' in relationship to the brake pedal and the master.  This will help with the amount of force on the pedal as well as the pedal travel.

Take the Ford Pinto for example (something I know a little about).  When Ford put power brakes in the Pinto they replaced the brake pedal with a longer one that had a longer distance between the pivot point (mount) and where the rod to the booster/master is.  This gives less of the mechanical advantage, and allows more pedal movement than if it would have with the other pedal installed. 

Years ago I installed a disc brake 'kit' on a 1963 Impala that belongs to a relative of mine.  The kit included a new booster and master, including the booster mount.  The kit also had a linkage assembly that connected the pushrod to the booster in a way that reduced the mechanical advantage from the brake pedal.  This set up made the pedal VERY hard to press, to the point it was unsafe to drive.  I decided that I needed to remove that linkage so I found the parts needed and installed a straight pushrod set up.  The car now has a very reasonable pedal feel as well as a normal amount of pedal input needed to stop the car.





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  • jrbe
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  • From: Central CT
  • Registered: 2009-06-23
  • Posts: 921

Re: Brake Feel and Booster

Have you considered a vacuum regulator between the intake manifold and brake booster? You should be able to adjust the amount of assist with one by regulating vacuum in the booster. I haven't tried it, but should work if you get a regulator.

Different pad compounds have better modulation than others. Your current pads may be more binary than modulatable. Have you asked the pad maker if they offer a pad compound with less bite and better modulation?

If the tires are skipping on the pavement it makes it easy to lock up. If your shocks aren't great for your springs or if you're running a ton of air pressure in the tires making up for less than ideal suspension, getting the suspension and tire following the pavement better will help if it's suspension / hard tire initiated lock up.

Modifying brake parts should only be done if you're comfortable fabricating something your life will depend on. Since you've driven it with the booster disconnected- you know what the worst case failure should be like., If something fails in the vacuum regulator mod, the worst case should be the booster gets no vacuum. Seems the safest and has the benefit of being adjustable - if it works.

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  • From: WPB Fl.
  • Registered: 2010-10-27
  • Posts: 1,571

Re: Brake Feel and Booster

While I have never tried a vacuum regulator I (as a master auto tech) would say this is a harder task then reducing vac flow,  the booster itself is a vacuum storage vessel as well as booster, crudely restricting vs carefully controlling incoming vac will give the driver full brake boost on first-second application then subsequent brake applications well be less and less boost, this will repeat every turn.

Moving the pivot point is the easiest cheapest way to be consistent.

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8 Reply by jrbe (edited by jrbe 2016-12-13 06:12 PM)

  • jrbe
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  • From: Central CT
  • Registered: 2009-06-23
  • Posts: 921

Re: Brake Feel and Booster

Deceleration happens every corner you'd need to brake at which means a source of vacuum. The booster is emptied (or filled if you think of it that way)  to "store" vacuum any time the engine decels or idles. It will empty the booster very quickly as long as there is no restriction.

A restriction seems like a bad way of trying to limit vacuum in the booster. A long decel would mean full vacuum would be stored. It would be very inconsistent.

Moving the pivot can work. It can also cause bind if the brake push rod can no longer fully travel the full length of the master cylinder bore. Depending on the design, moving the pivot could mean what would otherwise be spongy brakes (further travel than normal) are now bound up brakes. That's why I mentioned being comfortable fabricating something your life depends on. What seems simple and is not fully tested / thought through could be a big deal if something goes wrong. No one wants a Darwin award for modifying their brakes.

-Killer B's (as in rally) '84 4000Q 4.2V8. Audis never win?
  • From: Houston, TX
  • Registered: 2012-06-21
  • Posts: 1,608

Re: Brake Feel and Booster

If the pedal with no brake booster felt very firm and hard to press, then a master cylinder with a smaller bore should fix the problem while increasing pedal travel. If your car came with a manual brake option, the swap should be simple. If not, you may have a little engineering to do. If you haven't already, I would find the online forum for your specific car model and start getting familiar with the search function. If you don't mind spending a little more money, you could also try calling a company like Wilwood and getting a human being on the phone for advice.

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  • From: Staunton, VA
  • Registered: 2012-05-16
  • Posts: 606

Re: Brake Feel and Booster

Your foot/leg/mind is better calibrated for modulating pressure than distance. I've played around with a few different sized masters in my manual brakes converted E30, and a master that just locks up the wheels with a very healthy application of pedal pressure, works best for me.

11 Reply by derekste (edited by derekste 2016-12-14 08:15 PM)

  • From: Aurora, IL
  • Registered: 2012-02-06
  • Posts: 910

Re: Brake Feel and Booster

when you say "lock up", do you mean:
a- fronts
b- rears
c- all 4

in my experience, it's (b). a proportioning valve may fix this. heck, a proportioning valve may fix both A & B. as others have said, smaller master cylinder may help. if you can score some dual piston calipers (assuming whatever heap this is on has single pistons), that may help as well. a much smarter teammate once tried to explain this to me, but you have to consider the ratio of the MC bore to the piston bore if you alter either parts. prop valve for fine tuning.

hope this made sense.

edit:

vacuum check valves: are they any? should there be some? do they work? are they facing the right direction? I once pressured a brake booster on a turbo car. stiff!

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  • Registered: 2014-11-11
  • Posts: 148

Re: Brake Feel and Booster

I had forgotten I had asked this question here, found it just now with a cornucopia of answers. 

I do not believe this car (1991 accord) had manual brakes available.  I am considering a few options.  I think we should upgrade the rear brakes to match the fronts.  That might have the effect of getting the bias back to normal.  We upgraded our fronts with larger disks from an acura CL.  The rears from a similar CL might do the trick.  I think too much forward brake bias is a likely cause of the problem root problem.  derekste pointed that out, and I tend to agree.  I'm also going to look into the master cylinder and boosters for other hondas. Perhaps a similar vintage civic might have something that will work.   

Thanks for all the feedback.  I'm going to re-read and consider it all and I'll report back when we've picked our course.

13 Reply by derekste (edited by derekste 2016-12-15 10:11 AM)

  • From: Aurora, IL
  • Registered: 2012-02-06
  • Posts: 910

Re: Brake Feel and Booster

ukemike wrote:

I had forgotten I had asked this question here, found it just now with a cornucopia of answers. 

I do not believe this car (1991 accord) had manual brakes available.  I am considering a few options.  I think we should upgrade the rear brakes to match the fronts.  That might have the effect of getting the bias back to normal.  We upgraded our fronts with larger disks from an acura CL.  The rears from a similar CL might do the trick.  I think too much forward brake bias is a likely cause of the problem root problem.  derekste pointed that out, and I tend to agree.  I'm also going to look into the master cylinder and boosters for other hondas. Perhaps a similar vintage civic might have something that will work.   

Thanks for all the feedback.  I'm going to re-read and consider it all and I'll report back when we've picked our course.

In my Honda experience (1996 Civic LX), the stock rear drums were sufficient to still lock the rear up if you tried, even though we had super aggressive front pads. this was likely mostly a function of weight transfer.

With those CL front discs you mention: assuming you put a bigger (bore) caliper on there as well? Now your fronts will have less proportional clamping force, given the same MC. Putting the (presumably) bigger bore caliper on the rear may help, but 4 calipers with bigger bore on the same (presumably smaller bore) master will put you at a net loss (but also maybe be the sweet spot for pedal stiffness). switching to the CL master may help this, but you might be able to simply get away with limiting the rear axle pressure via a prop valve.

let us know what you figure out!

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