You might recall that, in the last (and only!) time I drove the car, the brakes were awful. With the pedal going most of the way towards the floor with little feel or resistance, and not actually doing any braking until just before the pedal hits the carpet. Quite terrifying, actually, and the more I thought about it, the more I was convinced that there was something wrong, rather than any inherent weakness with the Hako braking system.
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I did a bit of searching around the vintage Z forums and there were an awful lot of posts complaining of the exact same symptoms. After replacing or repairing the brake master cylinder the pedal goes all soft and long and the brakes hardly have any retardation at all. In most cases, it was blamed on something called a Reaction Disc, which is this striped, #18 thing in this diagram below:
This is a diagram of a 240Z brake booster (a Hakosuka one is similar in design but smaller), and the Reaction Disc is a 6mm rubber spacer that sits between the master cylinder activation pushrod and the internal plunger inside the brake booster. If you remove the brake mastercyl and dislodge the pushrod, then it’s possible for the little rubber disc to fall out, and into the depths of the brake booster. When that happens, you have 6mm of freeplay inside the mechanism, and so the first half of the brake pedal travel does nothing except bridge this new gap.
It sounded plausible to me, so I took off the master cylinder and measured the depth of its internal piston. Then I measured the depth of the pushrod inside the booster…and well what do ya know? 6mm gap! So I reached inside the booster with my finger, and yes, down at the bottom was something round and rubberlike. But after 20mins of trying to dig it out with my finger (you can only get one finger in there!) I bit the bullet and removed the brake booster.
Here it is, with the pushrod removed (the rubber disc goes behind the piston end of the pushrod). And after jiggling the booster around for a while, the little rubber disc flopped into position, and you can see it at the bottom of this “well” inside the booster:
Carefully put pushrod back in place before the little %$$@^ rubber disc falls out again…
And bolt the master cylinder back, all the while keeping an eagle-eye on the pushrod!
Reconnect the pedal, bleed the fluid,Ā and….PHEW. The pedal feels normal. With the engine off, the pedal goes firm within an inch or so of travel….basically like a normal car in other words! I haven’t driven it yet but the pedal feels good, and I am sure this will restore normal braking power to the Hakosuka….so now I can floor the accelerator pedal in safety!
When I removed the brake mastercyl, I was quite careful not to touch the pushrod (I was scared that there might be a clip or something holding it in). But you might remember that when I sent away the mastercyl for rebuilding, it took quite a few days before it came back, and during that time I was working on getting the accelerator pedal adjusted correctly. I think what I did was accidentally brush against the brake pedal when I was mucking around adjusting the accelerator pedal mounting. So the pushrod got nudged out a bit and the little rubber fell out.
We get a 4 day long weekend over Easter this weekend, and all of the necessary parts to install the rear seat belts have arrived. So I’ll do that, and remove the front struts for coilover conversion this Saturday or Sunday.
…and yes, now that I look at the pictures above, it’s really bugging me that I didn’t repaint the booster before putting it back!
whats the vent for?
^ Not a vent, just doesn’t have the vacuum hose attached. š
Nice catch btw! And yeah, sucks you didn’t paint it while it was out. Brake boosters, in general, are a big pain to remove.
Yes, it only occurred to me after I bolted it up that I’d just missed a golden opportunity to paint it!
I’m sure a Z is the same in that the nuts holding on the booster are obscured by the steering column and so you have to use the wobbly universal-joint socket to get them off…nd you get a headache from hanging upside down in the footwell for so long š
And yes that upwards pointing tube is the vac line that goes to the manifold. I just hadn’t reconnected it yet.
I had the exact same problem when replacing the master cylinder on my Colt, I ended up bleeding the brakes twice before I relised something was amiss, took the cylinder in thinking I’d buggered it some how, one dollar later and it was fixed, little bastard disc!!
Nissan has a long and proud history of playing jokes on the home mechanic š
For example, on an FJ20 engine, the distributor is hidden under the inlet manifold….and you can’t quite reach it, so if you want to renew the cap and rotor, it takes about 15mins of trying different positions and approaches until you streeeetch and then can get a few fingers onto the cap and remove it.
…and then you discover that some comedian designed the rotor to be attached in place with a tiny little phillips screw! š
Another joke they play is with the 4cyl Lseries engine (not sure about the 6cyl ver) but the intake manifold and header are held on buy a couple of bolts that bolt on underneath the header. No human being can get his hand up there! I have small hands and it’s a bitch and a half.