1976 Brake Pressure Switch

[doug73cs]

My 76 2002 has a Brake Pressure Switch in a housing with two lines entering from the MC and two exiting to the driver front caliper.

My dash brake warning light has been on at all times whether the parking brake is set and when the spade to the sensor is disconnected the brake light operates normally (comes on when I set the parking brake).

Does this thing do anything that can effect the operation of the brake system or is it just the hydraulic equivalent of a junction box? Any reason not to eliminate it with brake line? Anyway to test the sensor?

Thanks

[kubes]

Both of my 76s have it disconnect as the sending unit has failed, but has been let in place. My understanding (via reading and searching here) is that is a pressure differential switch, not just a plan pressure switch... though I never dug any deeper.

[fwardell]

Let me guess - you've just changed out the brake fluid or added to it? I had this same problem, and there's an easy fix. Look on top of the brake fluid container. There are a couple of wires leading from a switch on top. That's the fluid level switch. Near the base of those wires is a little reset button. Push it in for a second and hopefully - PRESTO - light is out. Let us know if that worked.

[doug73cs]

Fwardell:

Yes, I have bled the system but the reset does not fix the warning light - still on regardless.

I have a complete front and rear 320i upgrade using all new parts (Tii MC and SS brake lines at all four corners) and the brakes are still not even close (or safe). I have eliminated the pedal fade while holding the brakes on at the stoplights and there is NO fluid loss anywhere in the system. The only things not replace are the hard brake lines and this sensor unit.

I'm assuming there is still air in there in spite of multiple flushes and bleedings so wondering if this brake pressure unit is pooched and trapping some air. I'm still looking for an answer as to whether bypassing the sensor and housing unit is an option but I wold like to know how the thing works. Nothing in the Blue Books.

Doug

[jgerock]

Doug -

That looks like a residual pressure valve (shot in the dark), but the ones I've seen are installed on the rear brakes when 4-wheel discs are installed. Does your master cylinder/booster have the little brake light switch near the firewall?

What does the RealOEM description say for that part number (other than the generic wording like "valve, brake")? Wonder if it is described in the USA owner's manual supplement.

Have you tried to bleed the valve by loosening the switch while someone holds the pedal down? Maybe there is air inside.

[jgerock]

Doug,

I looked at both the Haynes and Blue book manuals and couldn't find anything about this pressure switch valve/junction. I found your illustration on Max's site by randomly selecting a '76 2002.

#1 is the actual valve/junction and # 2 is the switch.

01 SWITCH BRAKE PRESSURE 1 34321115861 N/A ENDED

02 SWITCH BRAKE PRESSURE 1 61311360940 $32.10

I also looked at my Owner's manual supplement (published in November 1976) and it only mentions the brake warning lamp on the dash.

Illustrations in McCartney's Restoration book doesn't show it.

Can you bypass it?

[kubes]

Doug,

Jim's info is good. The switch / sending unit is cheaper than I remember. As I need it also for my cars.

This setup #1, #2 is a pressure differential switch on on 1976 cars. It senses that this is different pressure in one set of lines vs the other. e.g. brake failure. I found this info I believe here, but I am not 100% sure where I read it.

If you have bled the brakes and checked for leaks then you might be able to just leave one of the connectors disconnected. Again that's how my cars are.. but for 30 bucks and maybe better though Blunt I'll replace mine now.

[doug73cs]

I'm either a genius or really stupid. I just reversed the spade connectors on the two terminals and the light went out. Still works when the parking brake is set so I'll assume this is not related to the brake function. My apologies for the fuss.

Now I just have to figure out why my brakes don't perform as they should. Next to check is the pedal linkage to the booster. This car was an automatic when I bought it and is now a 5 spd. Rebuilt the pedal box with Bill Williams post as a guide (thanks) so everything has been apart and maybe the linkage needs further adjustment. The pedal effort seems consistent from top to bottom but I run out of travel before the wheels are even close to locking up. Scary stuff.

Doug

[jgerock]

Doug,

Sometimes the little things cause the most headaches. Glad you fixed the switch wiring.

Since you re-did the pedal box, are you sure you adjusted the master cylinder rod to the correct length? If it is too short, you won't activate the brakes all the way (I believe the piston inside the M/C toward the front of the car is for the front brakes).

Do your rear brakes try to lock up before the front? I could be way off on this reasoning due to the diagonal circuits.

[doug73cs]

Doug,

Sometimes the little things cause the most headaches. Glad you fixed the switch wiring.

Since you re-did the pedal box, are you sure you adjusted the master cylinder rod to the correct length? If it is too short, you won't activate the brakes all the way (I believe the piston inside the M/C toward the front of the car is for the front brakes).

Do your rear brakes try to lock up before the front? I could be way off on this reasoning due to the diagonal circuits.

Jim:

By master cylinder rod, do you mean the rod connecting the brake pedal linkage through the booster to the MC as in the diagram? I don't remember seeing any adjustment in the MC - just the hollow metal sleeve that went into the booster.

I'll check the length of the rod to see if I get the full range of motion. I'm also going to try bleeding the MC in place. Pressurize the system and open the individual line connections and run some fluid out along with any air. Messy but hope that would help.

Thanks for the advice - I'll post any results.

Doug

[jgerock]

Doug - the bottom of rod "A" has a threaded end that connects to the pedal. I tried finding a good picture of this but cannot at this time.

[ingraham]

kubes is correct about the purpose of the switch (item 2) in the brake line manifold (item 1). When this switch detects enough of a pressure difference between the redundant front circuits, it _latches_ on, and that turns on the dash light. It stays on until you reset it manually.

You can easily reset the switch by pressing the little plastic plunger between the electrical terminals. The plunger may be covered by a rubber boot, and the whole thing may be covered in stealthy grease and dirt.

If the switch doesn't reset for whatever reason, disconnect one of its two wires to extinguish the dash light.

[swizman]

This is GREAT info, and I've searched to no avail for the pictures that were previously posted on this issue.  Brake warning light stays on after you've bled the system on 1976 cars.  I just about had myself committed trying to source the problem.  Ingraham has posted a little gem here.  As I recall from the previous post, ONLY on 1976 '02s, there is that little reset button hidden on the bottom of the brake pressure switch, or differential pressure switch, that you need to push back in in order to get the brake warning light to go out and start functioning normally again.