Javelin brake booster

[PaulRM]

Very intrigued by the period advert on page 29 of the current Jowetteer for the brake booster. What the additional cylinder seems to simply do is effectively reduce the bore size of the standard master cylinder. Presumably this would make the brakes a bit sharper - ie less pedal travel, but not easier on the foot in terms of pedal pressure. Thoughts anyone?

[PJGD]

Paul, I have just written an article on brake boosters, which will appear in the next By Jupiter.

[Chris Spencer]

I'm at a loss as to why you would want to complicate matters with the fitting of such things - A well maintained braking system that is correctly adjusted on any fully hydraulic system fitted to either a Jupiter or a Javelin is perfectly adequate - All the UK race modified cars (that currently race) which includes 2 Jupiters (shortly to be 3) along with a Javelin, race with standard original braking systems (with exception of the pedal boxes on some of them) They have no problems in race situations nor do any of the road cars that have been through my workshop that have factory original systems. I am however appalled at some owners ideas of a correctly maintained braking system with little regards to general maintenance let alone wear in linkages / rods & cables

[Keith Clements]

Concur with Chris on owners' brake maintenance, here is an article from 2000
https://jowett.net/jowettorg/jowettnet/ ... brakes.htm

I was so appalled by the state of some Le Mans 50th Anniversary traveller's brakes that I thought I would add this entry into the technical section to start it off.

But having vacuum assist brakes as fitted in period on my daughter's Javelin does improve braking as commented on by the one and only Sir Stirling Moss when taking him to his 80th birthday party. https://jowett.net/jowettorg/jowettnet/ ... s/mods.htm

Also fitting dual circuit brakes may offer some redundancy, particularly on long rallies, although I do not know anyone who has done this apart from Julian Crossley on his racing Javelin. Brake failure does occur quite frequently with even Chris having suffered such a failure in his Jav and I have on all my Jowetts at one time or another.

[Chris Spencer]

Yes I have had the brakes fail on my Javelin when the master cylinder failed, I've also had the brakes fail due to faulty electric brake pedal switch on My P6 Rover (fitted with a servo as standard) also had the brakes fail on Bedford TK truck whilst I was driving it along with a Raleigh Chopper pedal cycle whilst riding it - However my point is that a brake booster / brake servo wouldn't have made the slightest difference had they been fitted to any of the above listed.

[PJGD]

As it happens, the article on brake boosters that I have written was neither pro nor con on the option to fit such a device, it was largely about how they work and what some of the issues might be if one were to fit a booster.

One of the issues that I did not adequately cover was the propensity for brake failure. The Girling wheel cylinders on our Javelin and Jupiters are very basic and fundamentally similar to any modern drum brake "corner module", and thus one would expect high reliability given appropriate servicing and maintenance. The same cannot be said of the Jowett master cylinder which is unlike any modern cylinder. Because it works on the tension principle, it requires two dissimilar seals with the braking pressure developed between those seals. All things being equal, one would expect a device with two seals to be more failure prone than a single seal. Now true, virtually all modern master cylinders have two seals because they have a split system and so a failure in one circuit still leaves the other circuit active. With the "full hydraulic"system as opposed to the earlier hydro-mechanical system, a failure of either seal results in total brake failure. This frightening event is not unknown to Jowett owners.

Another aspect that I did not cover in my article is that it seems reasonable to suppose that the master cylinder seals are more likely to fail or at least have a shorter life if, in normal usage they are regularly exposed to a higher pressure than a lower pressure. A factor to consider is that when using a booster such as the Girling PowerStop unit the pressure at the master cylinder is approximately one third of that at the wheel cylinder. Thus for a given rate of retardation let us assume that the pressure at the wheel cylinders is 750 psi [52 bar], then the pressure at the master cylinder will be 250 psi [17 bar]. This is not the case for a modern braking system where the boost effort is applied in the linkage between pedal and master cylinder. While this feature can be put in the "advantage" column for the PowerStop, it is also true that the brake booster is a complex device with multiple potential failure modes, and this must go in the "disadvantage" column.