Problem after big brake conversion

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AC sports
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Problem after big brake conversion

Post by AC sports » Tue Nov 28, 2017 3:40 am

I recently had a friend of mine come to show me his wilwood 4 piston front brake upgrade.
Originally the car came with a single pot up front. Rears are factory discs, again single pot.
His pedal felt firm and rather under servo'ed.
A panic stop had the rears lock up and almost spin me around. IR thermometer testing as well as brake dust build up showed way more rear bias.
I questioned what else he'd had done to the brakes. He said he had a bigger bore master and booster installed also, and rear brake compensator removed.
Would it be possible the 4 pots up front need more fluid to work properly? If so would an inline wilwood restrictor/compensator in the rear line to limit fluid flow to the back help? I feel it might....but the pedal will still be rather hard?
Any thoughts?

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Re: Problem after big brake conversion

Post by turbo2256b » Tue Nov 28, 2017 7:34 am

wHAT master cylinder? What vehicle? Were caliper piston areas figured old vs new front. Rear piston area. He may need a proportioning valve.
Bigger bore master cyl can cause more pedal pressure needed.

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Re: Problem after big brake conversion

Post by AC sports » Tue Nov 28, 2017 11:46 pm

Car is a late 60 ' s alfa romeo with discs all around.
No idea what master or booster. They look to be from another vehicle in the line up. He mentioned master went from 19 mm to 21 mm bore. Booster is visibly bigger than original at least from the outside.
Pedal pressure I'm sure he can live with. It's the rears locking up that's the problem. I suspect it could be cause the 4 pots up front need more fluid now than the original pot did.
I'm pretty sure his installer ordered them out of a catalog and installed without any math.
Will a valve in line with the rear line help get the fronts to bite ?

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Re: Problem after big brake conversion

Post by joe 90 » Wed Nov 29, 2017 12:43 am

I had a similar problem many years back.
I upgraded the rear diff which had much bigger drums on it.
The front brakes were the original discs and the rear end locked up making stops really dangerous.

The cheap easy fix was to re plumb the remote booster so that it only boosted the fronts.


Modern brake systems are dual circuit and the way to do it would be with a proportioning valve.

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Re: Problem after big brake conversion

Post by turbo2256b » Wed Nov 29, 2017 10:08 am

iF ITS A DUAL master cyl it could be hooked up backwards. EXAmple Fords the rear bowel is for the front, chevy the opposite same with wilwood dual. even hooked up properly a proportioning valve may be needed.

Wilwood I BELIEVE HAS A FORMULA on their web site to calculate piston area

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Re: Problem after big brake conversion

Post by chimpvalet » Sat Dec 02, 2017 7:10 pm

The Alfa should have braking force apportioned something like 65 to 70% forward at the dry pavement limit. The rear circuit limiter, or proportioning valve, is used where the physical properties of the rear components are sized to give greater than 30 to 35% braking without. This gives lower pedal effort for light stops, more even pad wear front-back, and better balance when wet pavement lowers the limit of braking. So I would suggest right off that the limiter be reinstalled. Next, the increase in M/C bore would have increased pedal effort, possibly without any positive contribution. In theory, larger area in the front calipers could require a larger bore M/C to avoid longer pedal travel but that is never a certainty. Pedal travel, once free play at the pedal has been taken up, comes down to pad knock-back due to axle bearing slop, deflection in the caliper mounts, and expansion of the flex lines under pressure. It is my experience that an utterly solid pedal is possible using a large slave-master ratio as long as these factors are kept under tight control. So in addition I'd use braided S/S flex lines all around, keep the bearings at the tight end of tolerances, keep the fluid at top spec and revert to the 19mm M/C.

If you can offer figures for the piston areas, OEM and Wilwood, further analysis is possible.

Cheers,
Steve

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Re: Problem after big brake conversion

Post by chimpvalet » Sun Dec 03, 2017 6:45 pm

Not to get too far down the road to presumption, perhaps you could ask what the owner was trying to achieve in making those changes?

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Re: Problem after big brake conversion

Post by bigblockmopar » Mon Dec 04, 2017 6:53 am

AC sports wrote:
Tue Nov 28, 2017 11:46 pm
...
Will a valve in line with the rear line help get the fronts to bite ?
No usually not.
Compare the combined surface diamater between the previous setup and the current.
With calipers with opposed brake pistons on both sides of the disc, I tend to recall you only need to combine/calculate the piston surfaces on 1 side only.

I would also reinstall the proportioning valve if it had one before.

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Re: Problem after big brake conversion

Post by AC sports » Tue Dec 05, 2017 12:07 am

Upon further investigation. ...the previous mechanic removed the factory proportioning valve. Without it I agree with everything said here, and I think I'm going to be chasing my tail trying to fix this.

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Re: Problem after big brake conversion

Post by Calypso » Wed Dec 06, 2017 6:35 am

New front discs may also have race oriented pads, which may not work properly before they are up to temp. Which means rearward bias before temps are up.

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Re: Problem after big brake conversion

Post by tresi » Fri Dec 08, 2017 8:53 pm

Many proportioning valves make sure that that the front brakes start building pressure before allowing fluid to flow to the rears. Put the valve back in and see how it acts.

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Re: Problem after big brake conversion

Post by AC sports » Sat Dec 09, 2017 7:42 am

Amazingly the in line valve to the rear brakes, has fixed the problem of the rears locking before the fronts. The problem now is, not enough servo effect. Probably a combination of larger diameter master, lower engine vacumn from the 304 degree camshafts, and the twin webers suppling vacumn to the booster through one runner port only.

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Re: Problem after big brake conversion

Post by chimpvalet » Sat Dec 09, 2017 1:57 pm

Might be pointless trying to run a booster with the IR intake and what looks to be rather sporty cam specs. I am at work on a project Lotus with roughly comparable engine characteristics and will delete the vac booster while up-sizing the rotors as far as possible. Will be quite some time before I can report the outcome as it's mid-point in a total restoration.
To recap, I'd take the booster out of the equation, reduce the M/C bore significantly while making sure to minimize pad knock-back as previously described. There is pedal motion slop inherent in using vac boosters.
Not the least surprised at what you've found in reinstalling the rear line valve, it's as predicted. We could discuss further with some better idea of the caliper and rotor specs, by the way.

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Re: Problem after big brake conversion

Post by bigblockmopar » Sat Dec 09, 2017 5:34 pm

I would probably reinstall the 19mm master cylinder.
2mm more piston area means 20% more fluid displacement if I'm correct.
The 4 piston calipers might not have (much) more combined piston area than the original (single ?) piston brake calipers.

But if it's really just a servo issue, you could look into installing a vacuum reserve canister or an electric vacuum pump for the booster.

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