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Lost my front brakes this Saturday


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On my way home from an AOAI Chapter event, I suddenly noticed my brake pedal with more "play" than usual. A short while later, the play increased and it took about 3 times longer to stop. A quick inspection revealed the front brake reservoir was empty. I must have been rapid and recent because the sides and bottom of the reservoir were still very wet. I still had the rears so I limped home.

It appears that the source of the leak is at the junction box for the electric switch. The 1971 Avanti had completely rebuilt brake network with stainless steel lines about 1-1/2 years ago. Silcone brake fluid was used. Have any of you had this problem before with recently rebuilt brake systems and silicone fluid?

Thanks in advance!

Have any

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Hardest brake lines to flare successfully are SS. Take the leaking line out, if it is a line, and look for a crack in the flare. It's possible if there are none that it just worked itself loose but this is the downside of SS in brake lines. They also have a large upside so don't get me wrong, I like them

As for the fluid differences, silicon vs ester, I don't think it would have made a difference. I believe silicon fluids are lower in viscosity but probably not a factor here.

Secondly, check the switch, your's wouldn't be the first one to leak.


Edited by Avanti83
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Silicone fluid has advantages going for it...and it was factory fill by Avanti Motors starting in 1970. A downside to silicone fluid is that it's not compatible in any way, shape or form with mineral based brake fluid. It forms clogs and bubbles in the system and can give a pedal feel like the system is full of air (air bubbles) and fade away altogether. After so many years one may not know if the two type of brake fluids have been mixed in your car. Even if silicone fluid has exclusively been used, it's much more difficult to fully bleed air out of the system. On the plus side, silicone fluid doesn't absorb moisture like mineral based fluid does, doesn't hurt paint if spilled and can last far longer and in more extreme temperatures. Silicone fluid was originally developed for the military for use in all climates and conditions.

Are you talking about the proportioning valve just below the master cylinder with the low pressure warning switch or the distribution block down by the frame with the hydraulic brake light switch? If it's the brake light switch, many have reported issues with silicone fluid killing the switch after only a short time. Some have had good luck. Some of the problem may be due to replacement switches now are made offshore and are of low quality compared to past versions domestically made. They simply don't last as long.

As important as brakes are, I would suggest rebuilding or replacing your master cylinder, finding a replacement block where the problem was and completely flush the system with an alcohol solution before filling it with brake fluid...silicone or mineral based.

You also might consider installing a mechanical brake light switch...inexpensive and will last and last and be more reliable.

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Anyone know of a source for the "proportioning valve" below the brake booster (or specific info on what other vehicles used that same "valve")?

Actually, I do not think it is a proportioning valve, but rather a junction block with a switch that activates a dashboard light if pressure is lost in either the front or rear brake lines.

I found some Chrysler and Corvette junction blocks on eBay that are quite similar to the one on my '71 Avanti, but they are not exact matches.

Update 2/2/2015: it appears to me that the junction block is most likely a match to the '68-'69 Camaro block (Camaros got front disk brakes with the Z28 option in late 1967).

I haven't ordered one to check it to see if pipe threads are a match, but it looks very much like my '71 junction block.

Edited by WayneC
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I've also heard the proportioning valve referred to as a distribution block, but it actually does proportion the amount of fluid going to the front and rear brakes. The rears have to have less fluid pressure or they will lock up before the front. It's a fixed proportion, not adjustable. If you can't find the proper unit adjustable proportioning valves are easily found from Summit Racing and Jegs. You do have to do a bit of adjusting to obtain the best front/rear balance with them but once set, there's nothing wrong with using one.

If you need the exact unit, Myer's Studebaker or Nostalgic Motors would likely be good sources.

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  • 1 month later...

RQB3263....1981 Avanti II ...Im loosing fluid from front reservoir ...no external leaks that i can find...possibly going into booster then into engine intake.....Can any Avanti brainiacs provide me with a Mfg. and P?N for this mastercylinder ???

Im sure its an "off the shelf" unit ....thanks....BILL GREGG, Melrose, Fl....352-475-3190

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I can't say for sure this is correct but the interchange listed on Bob Johnstone's website is for a NAPA TS101379, which is supposed to be for a 1967 Ford Fairlane w/disc brakes. It's probably not expensive...check NAPA or Pep Boys, Auto Zone, Advance Auto, etc., for prices.

I'm not familiar with the master cylinder on your car...is the front reservoir the larger or smaller of the two? If it's smaller, it feeds the rear brakes and you might have a leaking rear wheel cylinder. That might not even show from the outside. If so, your rear brake shoes are probably saturated with brake build and would require replacement as well as rebuilding or replacing the wheel cylinder.

If you can find no leaks anywhere, then you might have a bad master cylinder leaking into the booster. Maybe you can unbolt the master cylinder enough to pull it away from the booster and see if it's wet in there.

No point in replacing parts until you know the source of the problem.

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