Break compensator

[fred]

Hello All, On occasion my 1978 spider is dragging the rear brakes! It goes away after i stop and let them cool down ! Both rears are doing it at the same time which seems odd to me. I've had calipers drag on other cars but never at the same time. Because i never drove the car with the original wheels and springs I have no idea if it was an existing problem when i bought the car. Now it has the Vicks lowering spring with 15 in rims and large front brake upgrade, which by the way is working great.
Because its both wheels, I'm starting to think it may be the brake pressure valve for the rear! Is it possible because of some minor geometry linkage change do to the lower springs the valve is getting stuck and does not release the pressure !? The car is in another state and i only drive it every so often when I'm there.
I'm tempted to disconnect the linkage to test it out. As is, the front dips pretty good with even a mild brake applied and as far as i can tell the rear wheels are not locking up even on a dirt road. so I believe the valve is doing what is suppose to do as far as stopping.
Forgot to mention I did replace the pads but not the rotors. The flex brake lines look fairly new but i do plan to replace them. i have all the parts just need time.
Thanks in advance for any insight
Fred
stay safe

[davidbruce]

HI Fred, one thing to check if you haven't already is the flex line from the body to the differential. It is often missed when the ones at the wheels are changed. I had one on my 73 that was almost completely plugged and probably original. Acted almost like a one way valve.

[SteinOnkel]

Salve, Fred

I think you are on to something. I would suspect the lowering springs change the geometry in such a way that the proportioning valve has less tension on it, but it could just be defective.

I would start by looking at the linkage for the valve and making sure everything can move freely without binding. If it is stuck, I like to use lubricants like Fluid-Film to unseize these parts and keep them in good working order for a long time. Forget WD40, that's yesterday's news.

Cheers
Steiny

[18Fiatsandcounting]

Fred, one relatively easy thing to check is to put a bleeder hose on one of your rear brake calipers, loosen the bleed screw, and you should see fluid come out when your able & friendly assistant presses on the brake pedal. Nothing new there. But, if you now progressively raise the rear body of the car (not the axle) with jack stands, you should reach a point at which that fluid flow shuts off. Raise the car an inch at a time while checking. This sort of mimics the "pitching forward" that the car would see during heavy braking, and if the fluid comes out normally when the car is level but shuts off when the rear is raised several inches (just a guess), then your brake compensator would appear to be working.

For safety reasons, I suggest jack stands rather than just a hydraulic jack.

-Bryan

[SteinOnkel]

For safety reasons, I suggest jack stands rather than just a hydraulic jack.

-Bryan

I would suggest a lift. My neighbor's racecar fell off the jack stands yesterday. Luckily, nobody was hurt.

Never trusted those things and your test although reasonable seems a bit sketchy in the securing the vehicle department.

[fred]

Thanks, because of this issue i now plan to trailer it back to my home town so I can work on it this winter and I will surely try your suggestions. its 200 miles away and i was really looking forward to a long autumn drive home, oh well.
Thanks

[18Fiatsandcounting]

For safety reasons, I suggest jack stands rather than just a hydraulic jack.

-Bryan

I would suggest a lift. My neighbor's racecar fell off the jack stands yesterday. Luckily, nobody was hurt.

Never trusted those things and your test although reasonable seems a bit sketchy in the securing the vehicle department.

OK, but which would you prefer: Being under a car supported by solid steel jack stands, or being under a lift that is dependent on hydraulic seal/hoses/pressure, etc. I'd go for the former. To be honest, it always creeped me out watching auto mechanics standing while working under a large hydraulic lift with the car above them. Creeps me out. All it takes is for a seal to let loose, or pressure to drop, or....? Creeps me out.

By the way, did I mention that this creeps me out....?

- Bryan

[SteinOnkel]

For safety reasons, I suggest jack stands rather than just a hydraulic jack.

-Bryan

I would suggest a lift. My neighbor's racecar fell off the jack stands yesterday. Luckily, nobody was hurt.

Never trusted those things and your test although reasonable seems a bit sketchy in the securing the vehicle department.

OK, but which would you prefer: Being under a car supported by solid steel jack stands, or being under a lift that is dependent on hydraulic seal/hoses/pressure, etc. I'd go for the former. To be honest, it always creeped me out watching auto mechanics standing while working under a large hydraulic lift with the car above them. Creeps me out. All it takes is for a seal to let loose, or pressure to drop, or....? Creeps me out.

By the way, did I mention that this creeps me out....?

- Bryan

It might be counter-intuitive, but hydraulic pistons don't fail catastrophically. If a seal blows out, they will leak down very sloooowly. Also, the professional grade hydraulic lifts almost always have a locking mechanism. On older models, you can hear it go clunk, clunk as it engages into the next notch while the car is being raised.

I have one of these:

https://www.youtube.com/watch?v=jvcVT-OxqvE

Now compare that to a flimsy harbor freight jack stand. Where the entire weight of the car is on two cheaply cast points of steel with about 2 sq inch of surface area. Oh and they make them u-shaped, so they are nice and wobbly. Gives you the option of being on a surface that is kinda not really suited for it or on things that will get bent. Nice.

Also, those can and do catastrophically fail:

https://www.nhtsa.gov/press-releases/co ... ack-stands

1.7 million defective units...bin that junk.

Cheers
Steiny

[DieselSpider]

Salve, Fred

I think you are on to something. I would suspect the lowering springs change the geometry in such a way that the proportioning valve has less tension on it, but it could just be defective.

I would start by looking at the linkage for the valve and making sure everything can move freely without binding. If it is stuck, I like to use lubricants like Fluid-Film to unseize these parts and keep them in good working order for a long time. Forget WD40, that's yesterday's news.

Cheers
Steiny

With a lowered suspension sans any modifications to the proportioning valve linkage that would keep the valve more fully open since it would be reading the rear of the car as heavy while raising the car would lead to having the compensator activate and reduce rear braking if the linkage wasn't adjusted to the new unladen height. On a seldom used car one would have to suspect a clog or internally failing rubber brake line or perhaps debris from a failed rubber line getting into an orifice such as in the compensator valve.

If one used DOT4 brake fluid and still has the older rubber components inside the brake system that are not DOT4 compatible this could also be a contributing factor.

Skipping brake flushes and stretching the schedule out past 2 years can cause corrosion through water or acidity in the brake fluid and also cause this type of issue. Some may justify skipping flushes by testing the brake fluid in the reservoir however that can give you a false sense of security since even though the fluid in the reservoir may be clear it can be all black and corrosive at the wheel positions where all the heat is generated.

Brake hoses that look good on the outside unfortunately may still be in bad shape on the inside and may be plugged or the lining shredded and acting as a one way valve preventing the brakes from being fully released and of course that includes the rubber brake hose between the frame and the rear axle which many tend to overlook. After having lowering springs installed the hose between the frame and the rear axle will have a more constantly tighter bend than before and may fail internally at that location if it was not replaced with a new fresh hose when the car was lowered.

A lot of things to look into to vet this out. Hopefully the answer will come swiftly once its put under scrutiny.

[18Fiatsandcounting]

Now compare that to a flimsy harbor freight jack stand.

I totally agree with you if we're talking about the cheap HF jack stands, or any El Cheapo brand jack stands. In my case, my jack stands were bought 4 decades ago and seem pretty robust. My real preference, if you don't have to remove the wheels, is to put the car up on ramps.

I heard of a guy working on a big American car many years ago, and the car somehow fell while he was under it, but he just managed to jam a long spanner wrench between the floor and the body, which likely saved his life. Talk about too close for comfort... Now that would definitely creep me out!

OK, I've hijacked this thread enough; sorry!

-Bryan

[DieselSpider]

Now compare that to a flimsy harbor freight jack stand.

I totally agree with you if we're talking about the cheap HF jack stands, or any El Cheapo brand jack stands. In my case, my jack stands were bought 4 decades ago and seem pretty robust. My real preference, if you don't have to remove the wheels, is to put the car up on ramps.

I heard of a guy working on a big American car many years ago, and the car somehow fell while he was under it, but he just managed to jam a long spanner wrench between the floor and the body, which likely saved his life. Talk about too close for comfort... Now that would definitely creep me out!

OK, I've hijacked this thread enough; sorry!

-Bryan

I had that occur after someone kicked the car calling it a piece of junk out of frustration just after I had crawled under to look at what they were having a problem dealing with. That's when I noticed the jack stands on the other side were not level and from that kick the 69 Cadillac Sedan DeVille (heavy frame and a 472 cu in V8 so about 5,000 lbs of car) starting going over. I jammed a ratchet handle between the frame and the plate of steel left from using the bottle jack on the soft asphalt which held the car long enough to get out. Now I inspect all jack stands set up by someone else before going under. Jack stands on asphalt need some kind of platform under them be it steel or a slab of pressure treated 2X10 or better.

BTW: If you jack up the car side to side when putting it on the jack stands it does not matter where you purchased them as you will many times end up pushing the stands on the opposite side over at an slight angle and will need to jack that side up again to re-set them to vertical.

You can get really cheap or very good jack stands at Harbor Freight as they have both. They have also recalled some that they found were not made as specified by the Chinese manufacturer so if you own some Harbor Freight Jack Stands you really need to verify whether they were among those involved in the recall.

Its never really off-topic the bring up shop safety relative to the task at hand.

[SteinOnkel]

All of this pales in comparison to something like the QuickJack from the video I linked.

Yes, it's $1,700. At the end of the day, you gotta ask yourself - how much is your life worth to you?

I wouldn't say HF recalled "some" jack stands - 1.7 million units sounds like almost all of them. As for Bryan's antiques...material fatigue is real.

[18Fiatsandcounting]

I definitely agree about the safety aspects guys, and DieselSpider, yes, that might have been your story I was referring to. I can't always remember who had which story.

In my case, the garage is a level concrete slab and I take pains to safely get my 2000 lb Fiat up in the air. I also use multiple redundancies to keep it safe, like large wooden blocks in addition to the jack stands, and leaving the jacks in place when I can (I have two 3-ton hydraulic floor jacks). I will admit to some concern what would happen if I were under the car during an earthquake...

As for my antiques, yes, my '69 and '71 spiders are definitely in that class!

-Bryan