Brake Bleeding/Pedal Height/Rear Calipers

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fp55scca
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Brake Bleeding/Pedal Height/Rear Calipers

Post by fp55scca » Tue Apr 30, 2019 11:19 am

--Ok guys, there does appear to be some confusion with respect to bleeding rear brake calipers, adjusting the parking brake, and pedal height on the Fiat 124 Spider. Let me throw a few things out here. Please read the complete post before flame throwing, if you must!

--(1) If the brake compensator is still installed in your Spider (and functional) you will need to support the rear axle with jackstands when bleeding the rear brakes. If the rear axle (with compensator) is left to hang during bleeding, the process will be next to impossible due to the compensator reducing fluid to the rear calipers.

--(2) Parking brake adjustment has no bearing on brake pedal feel or pedal height, unless your parking brake is so out-of-adjustment that your parking brake is ON. Only air in the system, a bad MC, or a leak in the hydraulic system will affect pedal feel or pedal height.

(a) Parking brake adjustment is totally mechanical. A cable is tensioned, which eventually pulls on the cam-arm of the rear caliper, which rotates a slotted-vertical shaft(spring loaded) in the caliper, which in turn, pushes the threaded rod and piston into the brake pad.

(b) Think of this adjustment as "being a little bit pregnant". For practical purposes, It's really either ON or OFF. Cable adjustment affects brake lever position (clicks), but only as the cable is tensioned further will it eventually move the cam-arm on the caliper, and this is what engages the brakes. The arm could actually move slightly, without engaging the rear brake pad, but only slightly. In time, your emergency brake cable will stretch, which will result in the lever needing more clicks to engage the brake. Readjustment is necessary to have fewer clicks.

(c) Without tension on the cable/cam-arm, the piston in the rear caliper is spring-loaded at rest, in the neutral position. This is why the rear brakes are called "self adjusting". Remember that the rear piston is mechanically attached to the vertical shaft, which is spring loaded with a series of spring washers to the neutral position. Only brake fluid can move the piston, or the parking brake mechanical connection.


--(3) You could actually bleed your brakes with the parking brake on; but, the reason you don't want to do that is....with parking brake on, this moves the rear brake caliper piston outboard, which will then allow more brake fluid to fill the chamber behind the piston. When the parking brake is released, this fluid will have to push back through the system. I've never done this, because I've always bled the brakes with parking brake OFF; but, I don't think it would lock the brakes. I think the excess fluid would be forced out of the weep hole(s) in the top of the brake fluid resevoir whend brake pedal was actuated. In any case, bleed your brakes with the parking brake OFF.

--(4) The reason you want the rear caliper piston oriented correctly (horizontal, with the reference line on top) is because there is a depression on the inner side of the rear piston (corresponding to the reference line), that allows brake fluid to flow more freely into the back of the piston when brakes are depressed. With the piston retracted, the fluid ports (brake hose and bleeder) are masked, but the piston does NOT stop the free flow of fluid, because the chamber is "stepped". Your brakes will still work if this reference line is misadjusted, but just not as effectively.

--(5) The reason your rear brakes can still work (to some degree) with a lot of misadjustment, is because the fluid chamber in the rear caliper is "stepped". The first 1" of the chamber is about the same diameter as the piston (the piston rides on an o-ring). Beyond that, the caliper chamber is a few millimeters wider at the back, and this is where the ports (brake hose and bleeder) are located. In essense, the rear caliper piston can never block fluid to the chamber....

--I hope some of this helps dispell some of the misunderstanding we've had from time to time. I have some pics I can post if 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: Brake Bleeding/Pedal Height/Rear Calipers

Post by lanciahf » Tue Apr 30, 2019 12:08 pm

Thanks Jim!
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Re: Brake Bleeding/Pedal Height/Rear Calipers

Post by 18Fiatsandcounting » Tue Apr 30, 2019 1:39 pm

Thank you for the nice summary, entirely consistent with my experiences, and I even learned a few things. No flames from me! :D

-Bryan
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Re: Brake Bleeding/Pedal Height/Rear Calipers

Post by fp55scca » Tue Apr 30, 2019 5:23 pm

--I believe a picture is worth a thousand words. For those less familiar, hope these pics will help!

--Using a flashlight, I illuminated the chamber of a rear caliper to highlight where the brake hose and bleeder screw ports lie, relative to the piston. A third pic is intended to show the "depression" inside the rear piston that enhances brake fluid movement when the piston is oriented in the caliper correctly (toward the top). The last pic reflects the groove in the rear caliper piston, and the "reference line" which should always be oriented horizontally, and to the top when installed correctly!

Rear Caliper Fluid Ports-1.jpg
Rear Caliper Fluid Ports-1.jpg (74.06 KiB) Viewed 355 times
Rear Caliper Fluid Ports-2.jpg
Rear Caliper Fluid Ports-2.jpg (52.88 KiB) Viewed 355 times
Rear Caliper Piston-1.jpg
Rear Caliper Piston-1.jpg (51.97 KiB) Viewed 355 times
Rear Caliper Piston-2.jpg
Rear Caliper Piston-2.jpg (51.12 KiB) Viewed 355 times
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: Brake Bleeding/Pedal Height/Rear Calipers

Post by miker » Tue Apr 30, 2019 5:32 pm

Jim Scurria: thank you so much for this summary.

At this point, one way or the other, I've run a couple of quarts of brake fluid through my system. I'm recruiting my better half's right foot to do it one last time, the old-fashioned way: push down pedal, open bleed screw, pedal goes to floor, close bleed screw, release pedal. Order: PSR, DSR, PSF (inner), DSF (inner), PSF (outer), DSF (outer).

Question: how many repetitions of pedal push are needed before one can say "enough already".

ONE MORE THING... I just realized that I am losing 1/4"+ of pedal travel when bleeding from the carpet in the car. I'm thinking that this could be a problem too. I'm taking it out for bleeding.
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: Brake Bleeding/Pedal Height/Rear Calipers

Post by 18Fiatsandcounting » Tue Apr 30, 2019 8:15 pm

I too employ the use of my wife (my better 80% in my case).

As for how many cycles of pedal pushing are sufficient, it depends on the car. For Fiats that I own and that have bled the brakes in the past, perhaps 10 cycles for each caliper. For cars that I don't know the history of and that spit out very dark colored brake fluid with indications of other stuff (particulates), I might go through two or three entire brake reservoir's worth of fluid before I'm done. Brake fluid is cheap, and once you have the car up on jack stands and the like, might as well bleed the brakes to your heart's content. Just make sure not to empty the reservoir dry, and thus need to start over (I'm embarrassed to admit how many times I've done this. Sigh...)

As for the carpet issue, I've never experienced it as I have the rubber mats under where the pedals are.

-Bryan
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Re: Brake Bleeding/Pedal Height/Rear Calipers

Post by bartigue » Tue Apr 30, 2019 10:21 pm

Excellent post
-----------------------------------------------------------
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Re: Brake Bleeding/Pedal Height/Rear Calipers

Post by spider2081 » Tue Apr 30, 2019 10:41 pm

I would like less free pedal before the braking is felt. Is the rod adjustment in the vacuum booster the adjustment for the free pedal travel.
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Re: Brake Bleeding/Pedal Height/Rear Calipers

Post by fp55scca » Wed May 01, 2019 12:04 am

miker wrote:
Tue Apr 30, 2019 5:32 pm

...I'm recruiting my better half's right foot to do it one last time, the old-fashioned way: push down pedal, open bleed screw, pedal goes to floor, close bleed screw, release pedal. Order: PSR, DSR, PSF (inner), DSF (inner), PSF (outer), DSF (outer).

...Question: how many repetitions of pedal push are needed before one can say "enough already".
-------------------------------------------------------------------
--Mike, if your brake pedal is going to the floor during your brake bleeding sequence, you either have an excessive amount of air in the system, or you have a leak somewhere in the hydraulic system. I believe you have a new MC installed, but a faulty MC would be another reason for excessive pedal travel. For now at least, discount the MC (new), and hydraulic leak, if you see no brake fluid puddles under the card. That basically leaves air.

--There are many techniques for brake bleeding with an assistant: Here's what I favor...

(1) Starting with a clear tube on the bleed screw (closed) with the lower end of the tube in a clear jar, partially filled with some clean brake fluid.

(2) Have the assistant press the brake pedal 2-3 times quickly (it does not have to go full travel down; half way is fine) and then holding the pedal while you open the bleed screw about 1/2 to 3/4 turn and observe fluid and air bubbles as it escapes. Close the bleed screw, and have the assistant rapidly release the brake pedal. Keep repeating this secquence until only fluid (no air bubbles) are observed flowing down the tube and into the jar. (3) Follow the same sequence your described: RR - LR - RF - LF . Be certain to check the brake fluid resevoir after every few sequences to be certain fluid does not go below 1/2 (my margin of error). Keep refilling brake fluid resevoir.

(4) If after you have done all 4 corners like this, the brake pedal is not firm and high, repeat the sequence at all 4 corners. If still not successful, you have a bad MC. Even if brand new out of the box, it is possible for one of the MC piston seals to roll.

--Mike, please keep us posted on your results. I am more than curious about your situation.
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: Brake Bleeding/Pedal Height/Rear Calipers

Post by fp55scca » Wed May 01, 2019 12:19 am

spider2081 wrote:
Tue Apr 30, 2019 10:41 pm
I would like less free pedal before the braking is felt. Is the rod adjustment in the vacuum booster the adjustment for the free pedal travel.
------------------------------------------------
--Dave, the short answer is that unless your acorn nut is grossly out of adjustment, then this is not a method for adjusting pedal travel. A properly adjusted acorn nut should be 1-2mm beyond the face of your booster where your MC mounts. I just use a straight edge across the face of he booster (MC removed) and eyeball it. You just don't want the acorn nut touching the MC rear piston statically, as this will lock your brakes.

--If you have excessive free travel: (1) air in the system [bleed brakes]; (2) a weak MC [replace]; (3) old brake hoses (ruber) that expand excessively [replace]. Steel braided brake hoses usually have a teflon lining that does not expand during brake operation, and therefore will usually result in a less spongy brake pedal.
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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