LJ78 vac lines and Brake booster (1 Viewer)

[Rukkh]

I've been tinkering away on my LJ78 for some time, egr throttle plate and associated vsv's went away some time last year and i've been trying (with little success) in the last month or so to make the boost bleed valve work properly.

And then a few weeks ago i replaced the alternator because the brushes were well below spec (about 4mm... explains my low charge voltage) and I'd planned to upgrade because I wanted a bit more amps for my new light setup.. Went with this one Alternator - 12V 100A

Noting that it came with a new front of alt mounted vac pump.

Since then i've been having brake booster issues, in traffic i would periodically get a hard pedal and the e-brake light comes on (apparently there is a vac pressure warning sensor linked to that light which comes on when you get <200mmHG of vac.) The only real hairy bit we found wheeling a few weekends back, i couldn't put enough pressure on the brakes to stop myself from rolling backwards.

I did a little poking around tonight with the vac gauge and was only getting 200mmHg out of the vac pump at idle and 500mmHG at higher revs (brake booster line disconnected but all the other inc). Looking further I worked out that when doing my egr ages back i've followed a picture wrong and had t'd the vac line which goes accross the crossover pipe into the turbo sensor.

Blocked it off and now i'm getting 500-650mmHg of vac out of the pump at idle, and the booster seems more reliable.

But I can still seemingly get the booster to go low vac and turn on the light by pumping the brakes. Sometimes (facing downhill) after pumping till the light comes on, and then holding it doesn't recover until i release the brake.
The problem is - I'm not sure if this is normal? It seems like a lot of pumping, but I hadn't seen it before, and it seems like the kind of thing you might see on a steep bit of track - i've stopped go'd a few backwards retreats before, but maybe i wasn't paying attention.

Have any other LJ78 owners seen this? Are you able to do a quick test for me next time you're on a quiet downhill road?

I'm struggling to convince myself that the boosted vac line was the only issue - its been like that for months, what changed?

 

[GTSSportCoupe]

I haven't had any of those vacuum problems from what I recall, but I'll give a quick test next time I'm out.

On a seperate topic, I can probably help you with your bleed valve. If you're not running over 14psi boost, then there is really no reason for it. If you want to increase or decrease your fueling, just use the screw on top of the spill control valve. It's a very easy way to adjust overall fueling. If you are running over 14psi, then you do need a bleed valve to keep the computer from going into fuel cut.

 

[jblueridge]

Do you have vacuum reservoirs? On my BJ74 there are two steel cans in line with the brake and clutch boosters' vacuum lines. The brake vac can has a sensor that makes that brake light come on.
Can you rig your vacuum gauge so you can see the effect of pumpng the brake pedal?
I installed a larger brake booster and now I get maybe four pumps before the light comes on.
It takes time for the vac pump to catch up.

 

[Rukkh]

There is a small resevoir, which as you say has the vac sensor in it.
I can probably rig up the vac guage relatively easily and that is definately on my things to do, although to road test it will be a daytime/weekend job.

It does sound like its normal to be able to out pump the pump so i may have just passed into paranoid territory.


Re: bleed valve. I'm just topping out a little over 14psi at the moment, but my EGT's are generally pretty good (450-550C pre-turbo on the highway, crawling up to 600C on a reasonable range - although with softer tyre pressures on trails i tend to slow down a lot up hills to keep it from rising too much). What i was getting was inconsistant effect of the bleed valve sometimes it worked, other times it didn't and there seemed to be an almost binary point in the adjustment of the speed controller. I suspect that having the vac line plumbed into the boost line may not have been helping, so i'm interested to see how it goes now that is fixed.

 

[GTSSportCoupe]

Haha, yeah, the vac line connected to the boost line certainly would have cause issues for the speed controller. It is a pretty fine adjustment though. I found the sweet spot was within a 1/4 turn of a adjustment.

Another option is to adjust the spill control valve too. I'd say an 1/8 turn out might get your temps safe. An intercooler makes all the difference in the world too. Dropped my EGT's 250F!!

 

[jblueridge]

To test my braking system vacuum I disconnected the clutch booster and put the gauge on that line in it place.
Then I just sat in one place with the motor running and revved to different levels. Then pumped brakes repeatedly.
I had my boosters rebuilt and those technicians were surprised that the land Cruiser pump made so much vacuum. They said US made cars needed far less for boosters to work properly.