38mm rear calipers - maybe not my best idea

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miker
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38mm rear calipers - maybe not my best idea

Post by miker » Wed Apr 10, 2019 1:30 pm

I definitely needed new rear calipers. The new ones bleed on both sides, the e-brake works, all good. Note, if you by these and want to use Speed Bleeders, you need the ones that are 39mm long.

Just like "everyone" does the brown wire fix, I recently ripped out the rear brake compensator. My car has progressive lowering springs, so the car isn't butt-high as it was when stock.

The idea for the 38mm rear calipers was to balance the braking to the 10" rotors in front with Wilwood calipers.

The car *definitely* stops with more authority now, but the D/S rear wheel locks when I stand on the pedal. I have not gotten the other wheels to lock yet.

I've ordered a new rear compensator valve (no point in putting one back on that's 40 years old), but I can't help but think that had I just put new stock calipers on the back I would have been just as happy and maybe I would not need the compensator.

By the way, I'm not using anything special on the rear pads - just Autozone Duralast - one good thing is that they don't have to be sanded down to fit.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 38mm rear calipers - maybe not my best idea

Post by lanciahf » Wed Apr 10, 2019 4:03 pm

Maybe not such a bad idea. Try loosening the e brake cable. Maybe the cable is pulling one side more than the other. Effecting braking.
Ralph DeLauretis
2013 Fiat 500 POP
1984 Pininfarina Spider
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Wed Apr 10, 2019 5:59 pm

miker wrote:
Wed Apr 10, 2019 1:30 pm

...The idea for the 38mm rear calipers was to balance the braking to the 10" rotors in front with Wilwood calipers.

...The car *definitely* stops with more authority now, but the D/S rear wheel locks when I stand on the pedal. I have not gotten the other wheels to lock yet.

...I've ordered a new rear compensator valve (no point in putting one back on that's 40 years old), but I can't help but think that had I just put new stock calipers on the back I would have been just as happy and maybe I would not need the compensator.
-------------------------------------------
--Mike, the concept of the 38mm rear caliper to balance 10" rotors is sound.
--From the description of what you have accomplished, I don't believe re-installing the rear bake compensator will fix your unequal braking situation.
--I can't remember all the background on this project, but did you replace All of the brake hoses when you did this job? You seem to have an unequal distribution of brake hydraulic pressure, and that normally happens with incomplete bleeding, or possibly a restricted brake hose, or both.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by miker » Wed Apr 10, 2019 8:37 pm

I have newer braided SS brake hoses. The MC and booster are new. The rear calipers are new, and the Wilwood calipers in front have a few thousand miles on them at most. I have not had a chance to find an empty, wide, dry road where I can stand on the brakes at 50+ mph in an attempt to lock all the wheels up. From a 30mph stop, I could only lock the DS rear.

Why wouldn't the rear compensator keep the rear from locking first?
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Thu Apr 11, 2019 2:21 am

miker wrote:
Wed Apr 10, 2019 8:37 pm
... From a 30mph stop, I could only lock the DS rear.

...Why wouldn't the rear compensator keep the rear from locking first?
------------------------------------------------------------
--Mike, I get your point, but I think you're following circular logic. You added 10" rotors with Wilwood calipers to the front of the car; and then removed the rear compensator and OEM rear calipers, and installed new calipers with 38mm pistons. Now you want to re-install a rear compensator to counter the larger rear pistons...?

--I think re-installing the rear compensator may just mask the problem. There is no logical way that one of your rear calipers should lock before the front calipers, unless something is faulty in the installation, and that should include calipers, hoses and brake lines. I recommend that you re-check and bleed all 4 corners again, and then be sure that all the new pads are bedded in properly.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by miker » Thu Apr 11, 2019 11:12 am

I'm more than willing to bleed the brakes again. I can get brake fluid at the grocery store when I shop with my wife, it comes out of the grocery budget that way vs. out of my "Fiat" account :).

I have every method possible for bleeding the brakes. What's your favorite? How much fluid does it take to really know you've gotten an air bubble out from nose to tail; in other words, when to stop? I will confess that I did not "bench bleed" the new MC. I could not figure out how to do that without making a complete mess.

Because I'm not a racer, I have relatively little experience with 'bedding' brakes in or knowing when they are in fact, OK.

This whole process reminds me that one cannot learn "skill" from a YouTube video.

Why is the rear compensator superfluous to begin with?

Thanks for hanging in there on this one...
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 38mm rear calipers - maybe not my best idea

Post by paulc » Thu Apr 11, 2019 4:05 pm

Actually I found the gravity bleed method works as well as anything.
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Thu Apr 11, 2019 7:39 pm

miker wrote:
Thu Apr 11, 2019 11:12 am
I'm more than willing to bleed the brakes again. I can get brake fluid at the grocery store when I shop with my wife, it comes out of the grocery budget that way vs. out of my "Fiat" account :).

I have every method possible for bleeding the brakes. What's your favorite? How much fluid does it take to really know you've gotten an air bubble out from nose to tail; in other words, when to stop? I will confess that I did not "bench bleed" the new MC. I could not figure out how to do that without making a complete mess.

Because I'm not a racer, I have relatively little experience with 'bedding' brakes in or knowing when they are in fact, OK.

This whole process reminds me that one cannot learn "skill" from a YouTube video.

Why is the rear compensator superfluous to begin with?

Thanks for hanging in there on this one...
-------------------------------------------------
--Mike, the rear compensator was a reasonable design in it's day, given the 4 wheel disc brake configuration. Remember, most other cars of that era, save for expensive sports cars, were still using drum brakes on the rear, if they even had disc brakes on the front at all! The rear compensator is ok, if everything is stock, and working well. But with age, the rear compensator can get sticky, and unreliable, and that's the last thing you want in an emergency stop situation.

--For brake bleeding, Paul's suggestion of gravity feed is a good place to start/test for air when brake bleeding has already been accomplished. I like to put the car on 4 jackstands, wheels off during brake bleeding, so I can move from wheel to wheel, and back again quickly, as needed. Always make sure the bake fluid resevoir is topped up. ( I use Valvoline synthetic Dot 3&4. Not expensive, and a very, very good brake fluid).

--Crack the right rear bleeder open, and just let it sit. If you are using SpeedBleeders, you will have to remove the Speedbleeder from the caliper for this check. If there is no air in the system (at least to the right rear) you will soon begin to see brake fluid seep out of the bleeder/caliper. For an initial bleed, this can take some time. I've even gone to lunch with a bleeder opened and a catch pan under it. You'll loose some fluid, but no matter, you will know that the fluid is moving through that circuit . If no fluid, you may have a problem.

--You can check each corner is this manner. Usually RR, LR, RF and then LF. If each circuit has been bled properly, you will usually see some fluid soon after opening a bleed screw. If you are not getting fluid during this check from all 4 circuits, you need to do a conventional bleed.

--There are many methods for bleeding. Here's what I generally do. Even with SpeedBleeders, it's best to have two persons. Top resevoir. Start at RR. I use a piece of clear rubber hose that will fit over the bleed screw and run down into a clean jar with about a 1/2" of brake fluid in the jar. Crack open the bleed screw with an 8mm wrench and say "open" to your helper. Helper then pumps brake pedal (full strokes) 3 times, and on the 3 stroke holds the brake pedal down and says "hold". Close the bleeder when you hear "hold". Release the brake pedal.

--Repeat this exact procedure as necessary. Air contaminated systems can take many, many repetitions. Each repetition of 3 pumps should produce air bubbles in the clear hose. Check the brake resevoir frequently to ensure you do not run out of fluid (as this will induce more air!). Eventually, the air bubbles will subside, and you will be pushing clear fluid out of the caliper, through the clear tube, and into the jar. Do all 4 corners this way (RR, LR, RF, LF).

--Depending on the success at each corner, and the extent of air, you may have to go back to RR again, and repeat until clear fluid is observed.

--Sorry for the length of this Mike. We can get into brake rotor and pad bedding-in separately. It's all to give you that good, confident stop when needed.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by miker » Thu Apr 11, 2019 9:46 pm

Thank you so much for the detailed procedure! BTW, with a new compensator, would you still be opposed to using it?
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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Re: 38mm rear calipers - maybe not my best idea

Post by fiatrn » Fri Apr 12, 2019 12:09 am

Under braking, the weight of the car shifts forward, and the front brakes do more of the work than the rears. To even out the braking forces, Fiat designed the compensator valve. As the rear suspension rises up a little under braking, the compensator slightly cuts the flow of brake fluid to the rear brakes, so they don't squeeze as hard and are less likely to lock up accidentally. When working properly, the compensator is a great design.

However, it relies on being clean and pretty and not crusted up and clogged, and it relies on a linkage that can get wonky and must be adjusted properly. When it isn't working correctly, it can cause too little fluid to get to the rear brakes, and they barely do any work at all. That's not a great situation.

'No compensator' might be better than 'broken compensator' but...

There is no reason to choose No Compensator instead of a Properly Functioning Compensator - especially for everday street use.

Many might argue, but they are wrong ;)
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