Spongy Brakes..... the penny has dropped

[pete3000]

I've been struggling with poor brakes on my 1986 110 for years, even after changing the drum rear axle to discs and going vented at the front, just about every part was renewed (one part at a time) and replumbed. The efficiency on the tapley meter at the Annual MOT "was just meets minimum effort". This was with standing on the pedal to get any braking effort.

Well everything went back in again recently when the chassis was swapped. The 1986 style (new but 2 times changed) brake master cylinder and pedalbox was removed and looked decidely worse for wear. So I bought a salvage 300tdi servo/ pedal housing and reservoir. Fitted all back up with a new 300tdi 90/110 master cylinder.

First thing i noticed about this was the front circuit has 2 fittings (1 for each caliper), rear has 1 port as before. But the diameter of the piston housing is much smaller than the 110 2.5NA disc/drum master cylinder.

After bleeding the brakes today, I can actually feel the brakes working without standing on the pedal. I even managed to lock the wheels on the drive. 🙂

Thinking about this i should have spotted a while ago that the laws of fluid transfer mean a smaller piston area can exert much more force than a larger piston albeit at a lower fluid volume.

Anyway problem solved, if anyone else is having similar issues after converting to discs it could be a mismatched master cylinder?

 

Pete

[western]

Plus 300Tdi 110 brake system doesn't have the PDWA in the circuits,

I retained my PDWA & the original spec servo & MC, since fitting stainless steel pistons to all 4 calipers my brakes are the best they have ever been.

[Northwards]

Hi guys,

Excuse me jumping in here - but it's a 300tdi 110 braking question.

I was having some work done recently which involved all new brake lines. Someone more experienced than me (which isn't too difficult) noticed that there was no PDWA valve on the bulkhead and suggested there should be one. I've since read that 90's had them, but not 110s; that it is the other way round; that ROW-spec 300s might have had them; or that they might have been spec for some overseas markets..... I don't know what to think now!

So the system was built up with a valve installed. I had brake issues (really spongy first pedal; good, solid second press). This eventuallyturned out to be a poor offside front wheel bearing. Now I have a decent pedal, passed its MOT OK.

But I have wondered about putting the system back to 'stock' - though with all the permutations above, it seems that 'stock' is something of a moveable feast.

From more experienced folk, what would the thoughts be? Would retaining or removing the valve make any difference?

Many thanks.

[western]

No PDWA on 300tdi, they were discontinued in 1991 rationalisation of brake parts, replaced by the low brake fluid float cap on master cylinder. 

[Northwards]

Thanks very much. 

In fact, I think I've confused the issue, and confused the purposes of the PDWA and the PRV. My mistake.

So, I guess what I really wanted to ask was whether the inclusion of a PRV would make a difference. Rather than write it all up again, its the same text as above, just with PRV wherever Ive written PDWA... 

Sorry for the mix up.

[pete3000]

I'd taken both my pdwa and the prv dip/dive valve out around 6 years ago, kept them just in case but never re-admitted them. As Western says the fluid loss switch is on the reservoir float cap on the 300tdi.

Pete

[western]

Prv wasn't fitted to 110 station wagons only hardtop & 90 models IIRC, it's meant prevent rear brakes on a lightly loaded or empty vehicle locking up before the front brakes, so you shouldn't have either or need to fit one. 

[Northwards]

Thanks guys. That is what I suspected. I'll be refurbing the front axle sometime in the early new year, so while the brake system is being disturbed I'll probably add this to the list.