308 brake failure warning valve

After a rebuild of the rear calipers on my 79 308, it has intermittently been showing the brake failure warning light. I have tracked this down to the check valve between the front and rear brake circuits (for those who don't know these, they are a simple shuttle valve that sit between the front and rear hydraulic circuits - any mismatch of pressure between the two circuits results in the brake failure light being activated).

After a lot of swearing and cursing, and the contents of one of the brake fluid reservoirs on the garage floor, I've got the valve out, and trying to work out if its retrievable. I found a picture on an earlier forum supposedly showing an exploded 308 valve (couldn't figure out how to reference another thread?), but its nothing like mine. From pictures I've found, I'm pretty sure mine is original. It is so crude!

The exploded picture shows a much nicer design, with a proper screw in switch, opposing steel pistons which are sealed with normal looking hydraulic cups, and springs at both ends to keep the shuttle centred. Mine is a smaller unit, and from the amount I've been able to disassemble, appears to just have a solid rubber seal at each end, with a steel piston (the shuttle) captured between these. The switch is nothing more that two contact strips, with a small gap between. The shuttle has a tang that sits between these contacts, and movement in either direction earths one of the contacts. There are no springs as such. I'm guessing that centering is just handled by the rubber seals at each end, and movement is allowed by controlled deformation of these seals. It's pretty nasty. The unit is assembled from one end, and totally enclosed at the other. My piston is pretty much over to one side (the enclosed side of course), and I haven't been able to move it with compressed air. Trying a few soaking techniques at the moment to see if I can free it up.

Has anyone had one of these apart? I'm guessing a bit how it goes together. I've got the end cap off, and the seal at that end out. It appears that the bore is stepped with two different seal sizes, maybe to match the differing bore sizes in the master cylinder and ensure equivalent pressures on both sides of the shuttle. The tang looks like it is too tall to come out with the piston? Any clues of how to get the tang and piston out?

Thought were to either drill and tap the piston in situ, to give purchase for an extractor, or drill the enclosed end of the bore (and subsequently tap and cap this) to give access to press the piston out. The risk here is damaging the seal. Any experiences, thoughts, ideas welcome!

If no success, I'll reassemble it without the warning connection which I sort out a replacement. Again, older threads have referred to a Volvo 240 unit, which is a fraction of the price (over USD 1000) of the replacement Ferrari unit, but has a slightly different layout. That's one option, the other is to make one (son with access to a glorious machine shop definitely helps here!)

Andrew
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Thanks - the reference you gave was the one I mentioned. Couldn't see any way of capturing the reference on my phone, but on the PC it was trivial!

A question on the Volvo unit. The Ferrari part is a simple differential pressure switch, and has no effect on the front and rear pressures. I believe the Volvo unit is a proportioning or bias valve, and I'm guessing its non adjustable. Does this cause any issues in the 308 (I'm guessing again, but I'd assume the Volvo puts more bias on the front than you may want with a mid engne).

Andrew

 

A bit more clarity. My local brake people list these under proportioning valves, but they are apparently the same as the Ferrari - a differential switch.

Just chasing up availability here (NZ), or will look at shipping one from OS. I notice there are aftermarket ones available (MTC) - anyone have any experience with these?

I assume these still use 10X1 flares? What size and type of brake lines are you using.

Regards

Andrew

 

I've finally got this resolved!

My intention in the short term was simply to replace the value with a couple of hard connectors to link the lines - a straight barrel for the rear brakes, and a T nor the front. I called into a brake shop that had been recommended to me, and had taken the existing valve in to size the connectors. The gentleman there took a look at the valve, and rather than running screaming for the hills, said he was confident he could get it apart.

He had to hydraulic the valve apart, but other than the seized pistons (there are two), the rest was remarkably intact, including the rather unusual seals. So now its all been cleaned up, bead blasted, repainted, reassembled and reinstalled, all for the princely sum of NZ $130 (about $95 US, 80 Euro).

For posterity, I got him to photograph a parts explosion before he reassembled the unit, which is attached (hopefully)



This has a few quicks;
- It has relatively few parts - a solid disc seal at each end, separated by a pair of pistons. One of the pistons has a raised centre to prevent them closing completely together, and in the gap is retained a formed wire clip that makes up the switch tang, My tang came out in two pieces, but because it is captured rigidly between the pistons and inside the valve cylinder, it doesn't matter.
- There are no springs as such to centre the tang. I'm guessing this is handled purely via deflection in the outer seals.
- The seals are a larger diameter than the pistons that go between them, As I understand this, the bore of the valve is stepped, and larger at both ends than where the piston sits in the middle. Because everything is inserted from one end, this means removing and reinstalling the far seal is fiddly, as it needs to go through the smaller piston bore. Who knows why it was made this way!
- the tang also extends past the piston bore, so the pistons cannot be simply forced out with hydraulic pressure at the far end, Rather the pistons have to be loosened up enough to first remove the outer piston, then the tang comes out the bore, and finally the second piston and far seal.

Probably not a lot of use to others around the world, but for those in NZ, I can thoroughly recommend Brad and Wairau Brakes in Auckland. Its great to deal with someone who sees something unusual as a challenge

Andrew

 

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