Brake issues - technical question

[DavidHill]

Morning,

After getting my Aprilia on the road after around an 8 yr and counting restoration i found that the brakes were really weak.  So have now taken all the drums off to investigate.  Have found the following: front brakes: drivers side shoes hardly move at all, passenger side shoes also hardly move at all and have a leaking cylinder. Rear brakes: Drivers side both shoes move well, passenger side one shoe moves well, other less so.

So can I conclude that the master cylinder is working ok, or could it be the issue causing both front brakes to hardly move?  (My car is a series two with a master cylinder labelled P25-30.)

cheers
David.

[davidwheeler]

I would suspect all the seals and renew the lot.  Get a full set from Omicron, take off all the cylinders, unseize clean and restore them and do the same with the master cylinder which is itself a very simple thing compared with modern ones.    I would renew the hoses whilst I was at it also.    You may also find the aluminium shoes are binding upon the steel pins?pegs? on which they sit because of subtle corrosion.    Waste not 8 years work for threeha'pence. 

[simonandjuliet]

Also consider sending the cylinders and M/C to "Past Parts" in Bury st Edmonds (from memory) to get it all checked over. They will measure everything accurately and assess what is repairable. Over the years many cars have had 1st ser cylinders added to 2nd ser cars because the later stuff has been more difficult to source.
They have done several sets for me and I've been pleased at both price and service

Happy New Year

[DavidHill]

thanks for the replies David and Simon,

I managed to take the whole Master Cylinder off the Aprilia yesterday and today after some battling with a screw that blocks the piston from coming out once the circlip has been removed from the end, i dissembled the master cylinder.  The piston itself looks ok- as do the two seals. But on inspection of the bore i noticed that there is some small corrosion at the far end of the bore...this explains why the rear brakes were moving, but not the front - as the far end of the cylinder is supplying pressure to the front brakes and that's where the small corrosion is!  so now have to see whether it can be re-bored and sleeved or source a new one from Cavalitto's/Omicron....I also plan to replace all the wheel cylinder seals - just in case whilst they are off the car anyway...all the bores look pristine on the wheel cylinders...will keep you posted on progress...

Many thanks
David.

[davidwheeler]

My master cylinder has only one outlet, on the end thereof.  Dual brake circuits were not, to my knowledge, employed on Aprilias.    Thus the pressure acts equally on front and back.    I still think your front cylinders/shoes are seized.

[DavidHill]

Hi David,
see attached pic of my Mastercylinder.  it has two outlets, the first one going to the rear brakes and the end one to the front. See also in the picture the brass piston from inside the cylinder. This has two seals and two pistons that are connected together via a shaft and a spring..so they can move independently a bit (not much)...so i think either one of the seals is not great, or there is pitting affecting just one seals, or the mechanism that allows the pistons to move slightly is not working? Does that make sense?

[lancialulu]

Just out of interest where does the master cylinder reservoir feed in?

[DavidHill]

yes good question...from the steel top of the middle pipe out of the cylinder, the brass part of that pipe takes the feed to the rear brakes...

[davidwheeler]

Your cylinder is different from mine then and is perhaps a later modification.   Mine (SeriesI/II) has only the single cylinder.  The diagram (Tav.27) is from the series II book.   I am still not sure how it works as all the dual circuit cylinders I have worked with have two pressure outlets and a separate inlet from the reservoir.  Very strange...  Perhaps there is a non return valve in the line from the reservoir?  Otherwise, the pressure would merely displace fluid from the reservoir.

[Kari]

The master brake cylinder of David Hill looks like a SABIF P25-30. Here you'll find a description of it and how it functions. Sorry, Italian language.

https://www.registroaprilia.it/wp-content/uploads/2017/05/La-pompa-dei-freni-SABIF.pdf

Its stated that there is one connection at the front for all 4 wheel cylinders and the connection on the centre top accepts the pipe from the reservoir.

I hope that helps.

Karl

[DavidHill]

Dear Kari,

thank you very much for pointing that out. My master cylinder is a Sabif P25-30 and the picture from the TAV does indeed show both front and rea brake lines coming off the same exit from the master cylinder, with the other fitting connecting the master cylinder to the brake reservoir. My rear brake lines were fitted to the reservoir point too, which is clearly wrong! I will have to fix that before refitting the master cylinder.  Many thanks!
David.

[davidwheeler]

Thanks for the Tav which I have put into the technical information thread.

[DavidHill]

I seem to be missing a small part in my front brake wheel cylinders - on the diagram part 28 seems to be a washer that distributes the spring load onto the seal. I am missing that and have talked to Omicron and they cannot supply one.  Any ideas on a) whether it is important to fit one , b) what the spec is if i need to make/source one?

[simonandjuliet]

I know that some are missing on my cars - and have been for a long time ..... try phoning Past Parts either for advice or the washer??

[chriswgawne]

The washers are I believe the same as those used on Aurelia (up to and including S4 B20 on the front) aluminium front and rear brake cylinders.
They are brass, perforated and cupped with little clips to attach to the steel springs and are there I presume to give some support to the rubber seals. Naked springs without washers obviously work but they wont be as kind to the seals or offer as much support as when the correct washers are used.
I have never seen such washers for sale on their own.
Chris