Rear Compensator, open or closed?

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San Jose Steve
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Rear Compensator, open or closed?

Post by San Jose Steve » Fri Dec 28, 2018 3:44 pm

I have installed a new rear compensator on the 124. Couldn’t get any fluid from the rear calipers. Car sitting on all 4 tires.... this morning I disconnected the actuating rod, but still get no fluid out. Is the valve normally closed? Does it have to be under tension from the actuating rod to open and pass fluid? I thought disconnecting the rod would do it for me. Nothing in my 3 different manuals really explains exact;y how the darn thing works. Can someone please give me a clue? Oh btw, all the rubber hoses are less than 5 years old....

Thanks, -Steve
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Re: Rear Compensator, open or closed?

Post by bartigue » Fri Dec 28, 2018 6:06 pm

Jack up the body - let the wheels hang. This puts the rod in the right position. Also make sure the hand brake is off.
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Re: Rear Compensator, open or closed?

Post by bartigue » Fri Dec 28, 2018 6:09 pm

And I find the system never feels right unless I bleed the rears of air and then work the hand brake about 10 times. I know, it shouldn't matter.
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Re: Rear Compensator, open or closed?

Post by fp55scca » Fri Dec 28, 2018 7:49 pm

--Errr Brad, I know we're getting older, but I think you got that one backwards? I believe letting the rear wheels hang will close the compensator, limiting the flow of brake fluid to the calipers; therefore, it's best to position the rear axle on jack stands when bleeding.

--Surprisingly, it's not addressed in most manuals, including factory shop manuals. The Haynes Manual 1968-1978 has a description on page 119, paragraph 2, that I believe to be correct. But even the Haynes Manual suggests starting brake bleeding at the front of the car. I disagree. I recommend starting at the furthest point from the master cylinder reservoir, which is usually the right rear; then LR, RF, and LF.

--AFAIK, the system works when brakes are applied firmly, and due to momentum and brake proportioning, the rear of car moves upward, and the actuating rod attached to the axle closes the compensator, limiting the amount (proportion) of brake fluid to the rear calipers. A rear caliper lock-up will contribute to a spin, thus the compensator is trying to prevent too much rear brake actuation.

--Personally, I never liked the system, even when new. I like it even less when the system ages and things get sticky. Many eliminate the compensator from the car, myself included.

--Steve, if you have a lot of air in the system, or if the rear hoses are collapsing (I know you said 5 years), it will be difficult to get the fluid to start to the rear calipers. You can check a hose by removing it and confirming that it has air flow. If pedal alone will not start the flow, try opening the right rear bleed screw, and let the car sit with the cap on the reservoir loose. Come back an hour later or so and check for fluid at the caliper. If this doesn't work, go back to pumping the brakes. Persistence will eventually win out.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
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1974 CSA Abarth Replica
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Re: Rear Compensator, open or closed?

Post by zonker » Fri Dec 28, 2018 8:33 pm

On stubborn hydraulic brake and clutch systems, I like to force fluid from the caliper to the reservoir. I use a clean squirt can filled with brake fluid and a section of hose and clamps for connecting to the bleeder valve.
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Re: Rear Compensator, open or closed?

Post by bartigue » Fri Dec 28, 2018 11:25 pm

I’m not commenting so much on the functionality but what I think is the correct orientation for bleeding.

Referring to the shop manual the bleeding procedure is done with the chassis suspended and wheels off, therefore the arm is at the maximum limit. I always bleed it this way.

I’ll also say I don’t think the compensator ever fully closes off fluid flow, so you should be able to bleed it either way.
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Bleeding rear brakes - axle position

Post by rridge » Sun Dec 30, 2018 10:40 am

The brake compensator valve varies rear brake pressure to prevent locking up the rear brakes in a panic stop with a light load. It should be fully open with the axle in its normal stationary position and it closes as the axle drops relative to the body. It is true that many mechanics bleed the rear brakes with the axle hanging at full droop. I think the manuals are silent on axle position but include pictures of the axle hanging down. As Brad says it may be that even a properly working compensator does not fully close off brake pressure with the axle at full drop.

It is also true that getting all the air out of the rear circuit on a 124 is a bitch, a problem never acknowledged by the factory. There are multiple issues like mis-adjusted parking brake mechanisms, leaky bleeder screws, etc. that contribute to the bleeding challenge for home mechanics. Getting as much pressure in the line from the master cylinder to the two rear caliper bleeder nipples while bleeding helps get a good and consistent bleed. Supporting the axle to keep a functioning compensator valve fully open while bleeding can only help the process.

See, I managed to write two paragraphs on rear brakes without ever touching on the compensator removal controversy. The search function can fill in the few owners who have not taken an firm and inflexible position on that one. Happy New Year.
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Re: Rear Compensator, open or closed?

Post by fp55scca » Sun Dec 30, 2018 7:57 pm

San Jose Steve wrote:
Fri Dec 28, 2018 3:44 pm
I have installed a new rear compensator on the 124. Couldn’t get any fluid from the rear calipers. Car sitting on all 4 tires.... this morning I disconnected the actuating rod, but still get no fluid out. Is the valve normally closed? Does it have to be under tension from the actuating rod to open and pass fluid? I thought disconnecting the rod would do it for me. Nothing in my 3 different manuals really explains exact;y how the darn thing works. Can someone please give me a clue? Oh btw, all the rubber hoses are less than 5 years old...
---------------------------------------
--Sorry Steve, we kind of left you hanging here debating jack stands, etc. So, first things first:

--When you installed your new regulator, did you check to be certain that you re-connected the brake lines correctly? The upper port is the delivery port out to the rear brakes. The lower port is the delivery port in from the master cylinder.

--Next, when re-installing or installing a new compensator, you must do a basic setup. I can summarize it here if you need me to, but it basically involves disconnecting the torsion bar from the axle, and adjusting the position of the regulator in the support bracket, by measuring the distance of the regulator from the chassis. Did you accomplish this? If not adjusted properly, it will not work properly.

--With reference to normal valve position: when the regulator is not installed, the piston in the regulator is extended and the valve is closed. When installed and adjusted correctly, the piston is held by the torsion arm with weight on wheels in the neutral position and fluid can flow freely.

--Finally, the only reference to axle position during bleeding that I have found comes from the Haynes Manual 1968-1978 Coupe and Spider, page 119, paragraph 2. "Bleeding the hydraulic system" as follows:

quote> When bleeding the rear brakes, place the jacks [stands] under the axle casing and remove the wheels. Do not use chassis stands acting on the body shell, otherwise the rear brake regulator system will be brought into operation and prevent the flow of fluid to the rear brakes. <end quote. (the bold text is in the manual for emphasis).

--At its very best, the system is rather twitchy to me. If not adjusted correctly, it will not work properly. When the regulators age, the pistons become sticky, and will not work correctly. We can each draw their own conclusions on this one! lol
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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San Jose Steve
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Re: Rear Compensator, open or closed?

Post by San Jose Steve » Mon Dec 31, 2018 11:46 am

Wow.....the valve is normally in a closed position fresh out of the box? I thought it was open until the actuating rod pushed the piston closed..... So the best condition to bleed the rears would be with the piston fully pushed in by the actuating rod. Damn.....that explains a lot. Wish that tip was in one of the 3 manuals I have!

So after ~60 cycles of the brake pedal, innumerable promises to my wife to not leave and my new found understanding of the compensators function, I now have brakes that lock up with a satisfying screech. (...but what an absolute “B” of a job it was!)

Rear axle supported by jack stands just high enough to remove the rear tires, compensator adjusted to spec, a capable if somewhat impatient assistant and some excellent advice carried the day. I just may make that New Years Day drive after all.

Thanks!
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Re: Rear Compensator, open or closed?

Post by fp55scca » Mon Dec 31, 2018 6:23 pm

--Congratulations Steve! I understand your "assistant's" frustration! One of my brides used to call it "playing pump the brakes"! If you find yourself without an assistant, you might try some "Speed Bleeders" P/N SB 8125L.

--These are 8mm x 1.25 thread pitch, and replace our bleeders directly. The "L" just stands for "long" so it's easier to attach a rubber hose. When the bleeder is cracked open, an internal spring and ball-cock keep the fluid in until the brakes are pumped. If you are really desperate, you can set-up your hose and catch-can (I use clear hose to see the bubbles) and then standing outside the car, you can use an appropriate stick (I use a cane with a rubber cap on the end) and push the brake pedal while viewing the bubbles. Going to the track without crew can be a trying experience!
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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