Vacuum from engine

[34ford]

Im having trouble with my brakes being hard. They have always been hard, as like with no booster. Its a new booster. As its in a project car, ive measured all piston sizes/master cylinder size and it all measures out.
We tested the vacuum coming off the engine (crown) and it was low. Is there something i need to do to get more vacuum from the engine? Thanks.

 

[cribbj]

When you say the vacuum coming off the engine is "low" can you be more specific, with some figures?

Engine vacuum at idle should be 15-20" of Hg in imperial units or 50-68 kilopascals. If it's not, then either the engine is pretty worn out, the sensing line you're measuring from is plugged, or the cats are plugged, etc.

Either that, or if you've modified the cams, and have high overlap cams, you may not generate enough vacuum for the booster. In this case, you may have to go to an auxiliary electric vacuum pump. I think that's what Zuffen did recently with a vintage racecar.

 

[34ford]

Thanks for the reply. I can't remember what figures i was getting but it was much lower then 50-68kilopascals.
Its a standard engine, so its not a high overlap cam problem. That leaves me with maybe the line where im measuring from is blocked. Measured from the rear port at the back of the manifold but with this unplugged the engine idles rough - so i don't think this is the problem either.
The motor did have 100kays on it when i bought it, but thats not alot. It doesn't lack power, no smoke etc.
When you say cats - you mean the catalytic converters? It runs a 2.5inch twin system. One muffler either side with one cat either side. What else can i try? thanks

 

[Zuffen]

Mick,

Double check the vacuum line isn't blocked and that the vacuum valve on the servo isn't installed the wrong way around.

It sounds like you're not getting vacuum to the booster but it's a bit hard to test.

Whilst it may sound silly have you tried running a line from another engine (your D/D will do) to the booster to see if the pedal gets softer?

Another option is turn it off and pump the brakes and see if the pedal gets harder to depress. If it does this will indicate it was getting vacuum. In which case it isn't an engine problem but a brake problem.

 

[sideshow]

i recon yr brakes are faulty or somehting wrong
ive never seen poor vacuum in over 100 std 1uzs
if motor runs ok then there is little chance of poor vacuum
also not common for leaks on that hose as its thick brake vac hose

also check yr one way valve is right way round

otherwise i would say yr booster is f k t

like zuffen said try using vac of another motor to tst yr booster

 

[34ford]

Thanks guys, Rod, you say "vacuum valve on the servo " whithout sounding stupid, where is that?
Sideshow,the one way valve - where is that? All i have is the fitting that bolts to the back of the manifold. Black hose from that to the booster. thanks again

 

[sideshow]

u need a one way valve or servo valve as zuffen has said that goes in series from booster to manifold

goto wreckers and take one of a jaaaaappppp car stupid forum cant say j a p
and im not even racist hehehee

get the ones that have a hose either side
they usually have an "E" makred on it with arrow too
e means enigne and arrow goes towards motor anyway one way valve shold allow
engine to suck from booster to manifold

if u dont have htis then ther eis no way for vacuum to form propelry in yr booster

 

[34ford]

Thanks sideshow. I'll drop down to the wreckers tomorrow and see what i can find. I thought the one way valve on the booster is all that was needed.

 

[sideshow]

yes all u need is a one way valve
have u got this if its the right way then u have problem elsewhere
can be anywhere between the booster and the manifold

and u need to be able to blow air from booster to manifold and not suck

 

[Zuffen]

Jim,

Remember this isn't a standard UZ instal.

The vac hose may be pressure line that will suck flat under vacuum. I've seen it before.

I'd be making sure the booster is OK before assuming it's dead and repalcing it.

Mick,

The booster most likely has a plastic valve where the vacuum line goes into it from the manifold (usually a white or black "L" shaped valve). If it has pull it out and try to suck through it both ways. If you can suck through it from either side it's dead and needs replacing. If it only sucks one way then all is good. Replace it but make sure the air can only flow from the booster to the engine. Not the other way.

Another test it to block the end of the vacuum line and run the engine. If it goes flat then replace it with the proper vacuum hose. It's cheap and may make a big difference.

Assuming the hose did suck flat it proves the engine is doing what it's supposed to.

 

[sideshow]

it should only suck shut if they used dodgey hose
proper brake vac hose is 10mm id and usually 4 to 5 mm thick

most backyarders use the closest hose they can find in the garage
but if u goto auto parts shop u will see brake vac hose is different to other hsoes

 

[sideshow]

one other thing i rememner from working on old datsun rally cars
im not to sure if new late model boosters have this but without it the pedal used to be rock solid like yr symptoms

they used to run a thing called a reaction disc
its a tiny rubber disc bout size of a 20 or 50cent piece
if u pull yr master cyl off u will see the piston or pushrod hanigng out of the booster
if u pull this piston out there should be a rubber disc about the size of the back of the piston/pushrod and it goes between the piston and where the piston locates in the booster
anywa just a thought so if your not to sure best ring place u got booster for to see
if has one necause i do remember that sometimes these discs would fall out if the pistons werent in there and they would sit in the bottom of the booster

 

[34ford]

thanks guys. Have only got back in the shed tonight, dam life keeps getting in the road.

The vacuum hose is not the proper vacuum hose, will replace this weekend. The hose im using did not suck flat, but i'll replace it anyway.

I've tested the one way valve, and that was fine. To pull the master cylinder off is a pain, so i'll do that over the weekend. In the mean time, i'll go to the brake place and see if they have one of those things you are talking about sideshow. Might even get them to test the booster.

Going to test the vacuum again on the engine, and let you know what figures i get.

 

[sideshow]

on late model engines with air flow meters if the vac is poor it greatly effects how it runs
so if yr 1uz runs ok then i pressume its not low vac due to poor motor
maybe low vac due to blockage in hose or banjo bolt in back of manifold

but like i said ive worked on iover 100 std 1uzs ive never seen low vac
id say bad booster

 

[Zuffen]

Mick,

Get the brake place to look at before pulling thr master cylinder.

Could be a simple fix and it will save you the grief of pulling it down.