Brake Sender Unit

**wbute** · 07-02-2014, 08:29 AM

Bimage.jpg
This is from the HJ service manual. The pressure switch is set the same for all brakes but discs have the proportion valve to prevent rear lock up.
You can't read it but it says
A brake differential switch is fitted to all models and illuminates a light on the instrument panel if pressure differential between front and rear brake circuits exceeds 120-170psi. On disc brake models the pressure switch is combined with a brake pro portioning valve.
So no doubt there many updates in the 10 years of production after this.

**SLR_dave** · 07-02-2014, 08:38 AM

Great work, but, doesnt discuss individual pressures for different body styles, (unless I read it wrong WB... it is sideways!). I think it just said that there is a proportioning valve on HJ if you have discs, it doesnt say much else?

**wbute** · 07-02-2014, 08:42 AM

Nah that's all but you need to look through all the manuals from HQ onwards for all the updates. I only have a couple.

**SLR_dave** · 07-02-2014, 09:55 AM

Yep, been doing that on what I have, I'm gonna bump my previous post though cuase its jumped off the page.

**SLR_dave** · 07-02-2014, 09:57 AM

Post bump. Is that rude?

Originally Posted by SLR_dave

Yep theres a good trail of developing information here, and I acknowledge all posters, but I was hoping we maybe had someone on the site who knew this backwards.

Or make it a project, heres why;

The question I was wondering was not so much as to when or why the wiring changed HX to HZ, but, why, for instance Jim would have the wrong switch. The obvious explanation is that either the valve, or the harness, or just the switch, is not original.

However, I would imagine it is as likely as any of these that a person would change the valve. (Say because really old brown sludge from the master cyl got pumped into it during a brake bleeed or something.) But this would mean the valve, not just the switch may be the wrong one.

And I have wondered about this before on at least one of my cars. Bearing that there seem to be at least four types of valve for HJ-HZ (and also used on LH-LX) which could be easily confused as they look close to identical, but, have ‘crack’ or valve opening points which it seems (following the links in my earlier post) vary from between 150psi and 450psi. This could alter braking performance considerably beyond optimal.

And this could of course be the case on any H series that an owner has not got an immaculate service record for – and whos got one of them?!! And we all want the best braking we can get – as it was designed.

So I was thinking it would be cool if we could gain a definitive list of the correct valve serial number for each body style, model, and brake options (drum or disc rear and (possibly) antilock HZ stato – if there ever was such a machine.

For those that followed the links in my earlier post (post #6) it seems to me that there were at least four types of valves, there seems enough information between these two webpages to conclude that that these were

P7256
P7259
P7305
and probably
P7306

I am not going to cite the member names from other sites in this post, as I don’t know the protocol for doing that, however there is some conflict of information between the two webpages I linked to about these valves and the models they were fitted to.

For instance two posts, (one member from each site) states that P7259 has a crack point of 350 psi.
Another member on the second site gives a more definative answer, gives body styles and series, and adds the fourth valve, but says P7259 has a crack point of 225 psi. For this reason I am not just posting what they said here. There is also conflict between that member and the other site on whether P7305 is 350 psi or 450 psi.

All seem to agree however, that there were different valves for different body styles with the same brake set up (discs front and drums rear), but their information conflicts or is not clear depending on which one you believe.

Neither of these sites specifically discuss the correct valves for vans (HX or otherwise).
One member discusses HX only having the twin wire for anti lock on statesman. And there may have been one (or more valves) coloured black, not aluminium silver

It seems believeable to me that the one for disc rears were the 450 psi version, however this is far from conclusive and there may have been more than four valves.

Ok so am I being pedantic? Personally I don’t think so, (lol) this is a question I have wondered about for ages, and the correct answers could improve braking and handling for any member who finds they have the wrong proportioning valve as much as RTS did.
Or a decent swaybar, etc. It’s a good question.

If I get time before the weekend, I’ll try to talk to the good brakes people down the road and see if they have some sort of H series proportioning valve book – that’s my current thought, otherwise I just dunno where to get this information and never have.

By the way Jim, what all this means for you, is that whilst your new switch will now fit your wiring, you still have a question (if it concerns you) as to whether or not you have the correct proportioning valve for your brake set up and body style and series. (i.e.; you could have a correct valve, or it could be one for four wheel drums…. ) But don’t worry I got an inkling there will be plenty in the same boat, or at least asking the same question.