I picked up my 73 XJ6 from a local repair shop today. You may recall this is the rust free California car I bought last spring. The car was suffering from a lack of brake boost at low speed. The pedal is extremely hard and required tremendous pressure to stop the car at low speed.
Despite the installation of a new booster and master cylinder, the car still requires tremendous pressure at low speeds in order to stop it. At higher speeds, the brakes seem normal.
What could cause a lack of braking power at low speed if the booster and master cylinder are working properly?
what are you using as a reference of “lack of braking power”? a new car?
The reason why I ask, is new(er) cars - like my wife’s Passat for example are over boosted brakes which takes little effort to put your nose into the windscreen.
do you exert the same amount of braking force with the car rolling without the engine running as you would if the engine is running?
This is to get a sense of how hard it is to stop the car. I too a '73 XJ6 (SWB)
When you first start the car; does your the brake pedal sink just a little?
if not, then there’s something wonky with the brake booster, because turning over the engine will create vacuum which should manifest itself through the brake pedal.
does the car run well? what is it idling at? should be around 750-800 rpm
this can be an indication of a vacuum leak which would effect the braking at low speeds though the engine would run like crap at an idle. it would be a massive vacuum leak.
when you put your foot on the brake, does the rpm change?
There is a vacuum check valve which isn’t located by or not the booster like the Series III or most other cars. providing this bit is stock, it’s located on a hose coming off the intake manifold just under or in close proximity of the front of the airbox, just before the snout on the airbox - near where the heat riser hose attaches to the airbox.
It’s a cylinder (shaped) about an inch or inch and a half long between two air vacuum hoses. If you trace the vacuum hose from the front of the booster along the bulkhead under the intake, you’ll find the one way vacuum valve.
Also, I wonder if a second vacuum two vacuum valves in series would cause this issue?
Is it possible the pedal linkage is binding or too tight?
The C27619 (part number) is actually located in the housing, and is the lever that pushes on the booster to apply the brakes.
There’s a plastic / rubber cap on the side of the brake pedal housing that provides access to where the booster and the brake pedal connect via the T shaped lever.
I’ve lost my train of thought on this one. Gotta get back to work…
I’m sure others will chime in soon.
The booster and master cylinder is simply not working properly, Lou - that speed is a factor just increases the mystery…
The booster has two chambers, divided by a ‘membrane’ with a commuting valve. The pedal rod acts on this valve and the master cylinder. Pedal released; the valve is open and manifold vacuum is present in both chambers…
As pedal is pushed, the commuting valve closes, and another valve lets air into pedal side chamber. The resulting air pressure moves the membrane and pedal rod - applying booster + pedal pressure. Increasing pedal pressure moves rod further, increasing the air pressure in the pedal chamber.
Brake force is proportional to pedal pressure - irrespective of road speed. A one-way valve between manifold ensures highest manifold vacuum is retained in the booster. As you release gas pedal to start braking - manifold vacuum shoots up for ‘best’ servo assist…
Four external factors;
The booster is not connected to manifold vacuum. It may be connected to ported vacuum - which has no/little vacuum in idle. Giving symptoms exactly as described…
Air is let into the booster via a channel coaxial with the pedal rod - there is a filter in there which may clog up. However, this will reduce booster assist in all situations.
The one-way valve may be defective/missing. However, as long as engine is running; there should be enough vacuum to operate booster normally at all speeds…
The booster hose is leaking. However, this will again give the same symptoms irrespective of speed.
First step; disconnect vacuum hose at the booster and measure vacuum with the engine idling - you should read some 18" Hg if hose is connected to the manifold. If not; connect the hose correctly - and check hose/connections fro leaks…
You may verify the one-way valve at the same time - stopping the engine; manifold vacuum should be retained. Rectify valve as required.
Listen in the footwell. There should be a brief ‘hiss’ during pedal movement as air is let into the booster. No hiss; either no vacuum in booster - or clogged filter.
If tests are normal - some internal fault in booster is likely/possible. The master cylinder is innocent; it is mechanically connected, through the booster, to the pedal. As Mark says; a rock hard pedal indicates ‘no booster assist’…
How old are the vacuum hoses Lou? Perhaps there is some partial closure or collapse is isolating the vacuum? My '72 had its breaked fully refurbished about three years ago: pistons, rotors, cylinders, booster, hoses etc. As a reference the pedal pressure required is just a little more than that required in our 2011 Subaru.
Collapsing hoses: yes,
Test the brakes without boost: yes,
And I don’t think there is a substantial leak (vacuum is the highest on overrun/idle) but my brakes bite harder when in 1st and overrun than at the same speed in third.
Shut off the engine and pump the pedal, how often?
How long does the boost last? If the brake gets hard right away there is a leak before the check valve…
usually it’s about one or two pumps. you’ll feel the difference in the pedal. When there is still vacuum in the diaphragm(s) the pedal will sink closer to the footwell, then eventually become harder to press, and farther from the footwell.
On my 73, by the second pump all of the vacuum is out of the booster, and the pedal is rock hard. Ok, maybe not rock hard, but firm.
As throttle is closed on the overrun, David; manifold may rise to 23" Hg - at which the overrun valve opens.
The faster the engine turns on the overrun the higher the manifold vacuum - and the lower the gear the higher the rpms. So your observation is certainly valid - the higher gear overrun may not yield the 23" Hg; the other factor in rpms is road speed…
The manifold having 23" - so has the booster…if it is connected to the manifold. If you now again accelerate; manifold vacuum will drop, and the one-way valve closes - the 23" is retained in the booster, ready for very good braking effect…
Nominally; as engine stops vacuum is retained in the booster by the one-way valve. With no vacuum supplied by a running engine; pumping the pedal, the booster will gradually lose vacuum and the pedal becomes harder.
The number of applications before the all vacuum is lost and the pedal becomes ‘solid’, varies a bit with the amount of original vacuum and how hard each push. Theoretically, without any leaks; vacuum is retained indefinitely. And since standard procedure is to apply brakes before starting the engine - one can make a conscious check each time…
All valid. If vacuum to the booster is obstructed and there is a small air leak somewhere to be found we have probably arrived at Jochen’s scenario haven’t we? The small leak continuously loses vacuum and in some scenarios the engine can’t supply a pressure differential high enough for canceling it out?
True, but how would that work on our cars: the brake booster is a large bore where as the ported port is like 3mm or so.
The tube collapsing at high vacuum (overrun and maybe idle) and opening back up at lower vacuum sounds most promising.
A vacuum leak large enough to overcome the engine vacuum at the booster would make the engine not run if the hoses and valve were in good condition.