8 replies to this topic

[MFM]

My LJ XU1 is fitted with a bathurst spec race cam (according to my mate who I bought it off - engine built by Bill MANN Brisbane ) and as it was sitting for 20 years unstarted when I  got it this year, I had the brakes and master cylinder reconditioned and then started it but I  appear to have minimal power assist brakes particularly at idle.

 

The car is not registered yet so can't take it for a drive, but I was told to test the XU1 booster for faults, pump the pedal several times then keep the pressure on the pedal and start the car and the pedal should depress when the engine starts. When I did this the pedal depressed about an inch or so after the car started.

 

Apparently that indicates the booster is working ok. Whilst the car is idling I pump the brakes several times and the revs increase. I also assume this means the vacuum is working. I checked the original albeit hard vacuum hose from the booster to the engine and its from 1972 but still seems to be sealing. I've only backed it out of the driveway and back into the garage and the pedal feels like there is no power assist. A reliable mechanic mate says that cars with high lift cams have no vacuum hence why the brake pedal feels hard. He also said that GTHO fords had the same problem but Ford fitted a vacuum cannister specifically for this problem at idle.

 

Do any of the forum member have this problem?

[S pack]

Mark, at idle is when manifold vacuum is at its greatest. If you don't have much vacuum assistance at idle you will have nothing at higher engine revs.

 

Best to get a vacuum gauge and test the manifold vacuum to see exactly what you have.

[grumpy xu1]

Mark, at idle is when manifold vacuum is at its greatest. If you don't have much vacuum assistance at idle you will have nothing at higher engine revs.
 
Best to get a vacuum gauge and test the manifold vacuum to see exactly what you have.

Spot on Dave, the lobe separation angle is what causes the less brake vacuum, the bigger the number the more vacuum, if it's say 104° the brakes will be ordinary. Check all the vacuum lines & clamps first. We had a friend's lc with a chev & when we bought it, it had no clamp on the vacuum line & the guy couldn't figure out the brake problem, we fitted clamps & walah brakes have vacuum instantly. Maybe check the back seal of the booster to, they usually hiss with a leak, if you can hear it over the engine. Keep your factory vacuum hose for display purposes, because no one in Australia sells hose with the grain pattern at all, i think it's 9/32" in size by memory & check that the check valve going into the booster at the end of the hose is working, you might be lucky enough to get a pbr 1 through a brake & clutch shop. Gary.

[V-SLR5000-P]

You need at least 15" inches of vacuum at idle for the booster to work properly. A vacuum tank is unlikely to help if you are below this as it can only store the maximum vacuum the engine is creating and cannot make more vacuum.

Due to low vacuum I run an electric vacuum pump to the booster, other alternatives are a hydroboost or a diesel alternator with a vacuum pump.

[MFM]

Ok thanks for this info. I did check the vacume hose clamp at the engine (tightened it) but not the other end at the booster as was fearful of braking the white plastic grommet, well more fearful of trying to find another one if I did break it by prying it out of its hardened rubber seal . I wasn't aware that this gromet was a one way valve? reconditioning the booster doesn't worry me, but don't want to go to that bother if its not the booster. Could also be that I'm comparing the pedal response to a modern car which I shouldn't. That's very interesting what you say about the vacuum should be more at idle. The pads on both front and back were like new but 25 years old. Could the good old asbestos pads need changing?

[RallyRed]

Unrelated to your present issue...but FYI................................ regarding the rear brakes - the hose from the body to the rear axle assembly is notorious for just blocking itself up with crap and swelling over the years. i.e. the brakes seem bodgey, as you have no rear brakes at all, as the flud never gets to the wheel  cylinders..

Cheap and easy to replace. 

 

My LC was also laying around for years, and the brakes were never great, got front calipers o/hauled and replaced rear wheel cylinders while I was at it....the brakes were better, but not great...then determined ( was advised?) that the hose was prob. blocked.

i.e the brakes are wonderful, if only the fluid could have got there.  New  hose and all good.

 

Datto is the guru on this stuff

Edited by RallyRed, 19 October 2018 - 06:19 PM.

[MFM]

Col you are spot on. My car had all the original brake hoses and when  we did the calipers and replaced the rear wheel cylinders and went to bleed them the rear passenger wasn't getting fluid and sure enough hose was swollen. Ended up replacing all 4 hoses. Couldn't get the same textured rubber line as the original ones had so had to make do with normal looking hoses.

[rodomo]

at idle is when manifold vacuum is at its greatest.

Not quite right, manifold vacuum is greatest on deceleration.

[S pack]

Not quite right, manifold vacuum is greatest on deceleration.

Thanks Rob, that makes perfect sense. Learn something everyday.