Packard Forum Index » V-8 (1955-56)

56 400 with 55k miles. Just waking it after a too long sleep so all the brakes, fuel system etc etc. have been brought up to snuff. Carb rebuilt, ignition in good shape and a nice new vac advance. Engine runs passable and valve train is quiet.

On the first test drive the brakes were terrible--almost no assist. No wonder--only 6 inches of vacuum that went to ten if you blip the throttle. Also, # 8 had only 60 lbs compression that did not come up if you oiled the cylinder.

OK, so off with its heads for a valve job, and a new timing chain as late valve timing can cause low vacuum. Chain was very slack but it had not jumped time yet.

Buttoned her back up today and went for a depressing test drive. Idles smooth, powerful out on the road, starts nicely--but still only 6 inches vacuum. Timing is set at 10 degrees and there are no vacuum leaks--have tried closing off all connections and spraying around every conceivable gap.

Ideas are earnestly solicited. Can't give it to the customer with not enough vac to assist the brakes.

Stupid question but is the same 6" measured at both the brake source and the WS wiper carb source? Was wondering if there is any remote possibility the brake check valve could be stuck in an all but closed position.

If all conceivable engine vacuum leak possibilities have been explored, how about a leaky booster diaphragm?

If that's out of the question, I would consider a possible vacuum leak from an ill fitting intake manifold, particularly on the bottom oil galley side where it would be hard to detect. Were the heads milled? That could change the alignment and fit of the intake.

Measuring the vac at the back of the carb before the check valve, and with the booster plugged off.

Leaking intake or blown head gasket. Perhaps the issue with the low compression on cylinder 8 is related.

Take vacuum readings at MANIFOLD not carb.
Secondaries of 4bbl carbs can hang open causing low vac.
Using prudent and reasonable caution squirt gasoline around potential vacuum leak areas. If engine rpm increases then a vacuum leak exists at the point to which gasoline was squirted.

Validate timing marks on harmonic balancer. Use a small bottle filled with oil and a hose connected to #1 spark plug hole to check accuracy of timing marks.

Ross, that's a bit of a stumper. Valve timing does seem a likely candidate but you've already checked that. The vacuum is far to low to be caused by one cylinder with poor compression, plus the vacuum reading might be expected to be jumping around and not steady.

I'm sure you've already checked for leaks enroute from the check valve to the power brake booster. Have you considered that the manifold vacuum might be pulling against an open end enroute to the crankcase vacuum pump? If you haven't already done it, you might pull the rubber hose at the firewall from the crankcase vacuum pump and block the inlet to the little plastic junction block and reevaluate.

It seems a pretty remote possibility, but any chance that the intake manifold was off the car in the past and dropped/damaged - a crack perhaps in the base or elsewhere?

OK, I knew it couldn't be rocket science:

Secondaries hanging open--return spring was off its little hook. And a goodly leak at the booster which was not audible. Phew.

Better cheque the secondaries close. They tend to crust up on cars that do not see much high speed operation over 60 mph or more.

Also, do the idle MIXTURE adjustment screws have little to no effect for adjustement?????? I'm assuming that's a Rochester 4bbl and not a WCFB.