Help! mastercylinder too small?

BradGTV · October 19, 2010, 07:27:11 PM

Hi Guys,
last week i put a set of volvo 4pot calipers and rotors i had onto my 83 GTV.
all went well and everything bolted up nice. i did the bearings and seals and re-greased everything.
then it was time to bleed. first of all the mastercylinder that was currently in the car broke its seal and brake fluid leaked out everywere. put the spare one from my 85 GTV on and fixed the problem. then started bleeding again. a mate and i bleed the whole system nine times! all air is out and the pedal feels like crap! a mate suggested the master cylinder could be to small for the 4 pot calipers? and on his VL Turbo had to make the upgrade to get rid of a sponge pedal. to test this we clamped of the front brakes using brake line clamps and then tested the pedal, it felt great! so we came to the conclusion that it must be the master cylinder no being big enough.

can anyone shed some light on the issue? when putting the volvo calipers on does it require a bigger mastercylinder? does anyone know of a upgrade for the mastercylinder?

cheers, Brad

Duk · October 19, 2010, 07:39:06 PM

When I put Volvo calipers on my 75 I had a pedal that felt crap aswell. But the brakes worked and I took it for a tentative drive. After a fairly short period of time they started to feel really good. A nice firm and reassuring pedal.
I can't say for sure how and why they got better, only that they did.

BradGTV · October 19, 2010, 08:26:53 PM

hi duk,
thanks for that,
my brakes dont feel good, but when i reversed off the hoist and tested them they gripped really well. i will do as you did and see how i go.
i plan to take the car out to mallala this saturday, so hopfully i can get a fix to the problem asap!
cheers, brad

Storm_X · October 19, 2010, 08:49:43 PM

Quote from: BradGTV on October 19, 2010, 08:26:53 PM
hi duk,
thanks for that,
my brakes dont feel good, but when i reversed off the hoist and tested them they gripped really well. i will do as you did and see how i go.
i plan to take the car out to mallala this saturday, so hopfully i can get a fix to the problem asap!
cheers, brad

is ther a open track day this saturday morning arvo ??
i am going to be there from5pm onwards.

BradGTV · October 19, 2010, 09:41:39 PM

yes open, for cars and karts, from 9am till the drifts start.

bt46 · October 20, 2010, 05:03:16 AM

Brad you may already know this trick and it might help.
Remove the negative batt terminal so your batt doesnt drain.
Using a piece of wood or a steering lock between the brake and drivers seat jam the brake down with moderate force.
Crack the cap on the fluid reservior.
Leave overnight.
This will ensure you have no stubborn air left in the system.
Works wonders on the clutch system as well.

I dont know anything about the volvo calipers but I think they have two bleed nippels that need to be bled together like the rear calipers. Also each servo unit seems to have a mind of its own and can really change the pedal feel but thats just a consideration as I asume your brakes felt ok as standard.

As an aside I have a one man bleeding tool that uses compresed air to draw fluid through the lines. That works a treat esp on the clutch system which can be a pain to bleed.

Storm_X · October 20, 2010, 05:48:12 AM

Quote from: BradGTV on October 19, 2010, 09:41:39 PM
yes open, for cars and karts, from 9am till the drifts start.

do you know the price to watch ? is it $10 same as drifts ?
thanks chris

BradGTV · October 20, 2010, 07:39:38 AM

thanks for the suggestion bt46, ill give it a go.
storm - watching is free

Duk · October 20, 2010, 01:44:23 PM

Quote from: bt46 on October 20, 2010, 05:03:16 AM
Brad you may already know this trick and it might help.
Remove the negative batt terminal so your batt doesnt drain.
Using a piece of wood or a steering lock between the brake and drivers seat jam the brake down with moderate force.
Crack the cap on the fluid reservior.
Leave overnight.
This will ensure you have no stubborn air left in the system.
Works wonders on the clutch system as well.

I dont know anything about the volvo calipers but I think they have two bleed nippels that need to be bled together like the rear calipers. Also each servo unit seems to have a mind of its own and can really change the pedal feel but thats just a consideration as I asume your brakes felt ok as standard.

As an aside I have a one man bleeding tool that uses compresed air to draw fluid through the lines. That works a treat esp on the clutch system which can be a pain to bleed.

That's an excellent idea about letting it sit over night with the pedal pressed.
I was able to bleed my Volvo calipers with the highest nipple. Obviously Brad made sure he put the correct caliper on the correct side of the car

.

Bob Morey · October 20, 2010, 09:12:17 PM

Hi guys, I had a similar issue on my '76 GT. I fitted a master cylinder with a 24.0 mm bore, the original was 19.0 mm bore. The replacement cylinder was identical in all respects except for the bore. I haven't kept full details but can find out this info if you are interested.

BradGTV · October 20, 2010, 09:17:16 PM

thanks bob, i recon i will upgrade the mastercylinder.
i have read 164 MC is a good upgrade? also one of a BMW? can anyone confirm which model and year?

Mike · October 21, 2010, 12:00:28 AM

It stands to reason that if you increase the caliper piston bores and hence the volume that would be required to move the pad 'x'mm then the master cylinder would also need to have the bore diameter increased in order to keep the same relative travel on the brake pedal that you have been used to.

I like the idea about the block of wood...it really works? I ALWAYS have issues bleeding bloody Alfetta brake cicuits....drives me batty.

MD · October 21, 2010, 08:47:01 AM

BRAD,

Just to reinforce the advice from Duk. The correct orientation of the calipers when mounted must ensure the bleeder nipples are at the top. If they are not, you will never get the air out of them. Don't laugh, it's been done before !!

The second point is that the bore size of the master cylinder is not responsible for your "crappy" pedal assuming you are saying it is not firm. This is due to two things. Firstly the residual air in the system and secondly from the firewall flex and transfer bar set up. That's why the pedal feel of a 105 is always different to a 116.A small bore of the MC is only going to affect the stroke length (pedal travel) but not the feel. Larger piston sizes require a large bore MC.Try a 25mm one.

To correctly bleed the rear callipers you need to bleed them both simultaneously.

You can make a home made pressue bleeder kit with a spare reservoir cap fitted with 2 hose nipples a couple of metres of plastic tube, a bike tube lock on valve clamp and a spare tyre filled at 10 psi. If I can find mine I'll post a pic. Haven't seen it for a while because I use a commercial product to do the job now.

Mike · October 21, 2010, 10:41:19 AM

yup, agree with MD, I assumed you were correct when you said that you had all the air out of the system...I guess the truth is you don't know that, so it's likely you have both issues, too small a reservour and air in the system. I certainly would not live with a hard pedal and lots of travel due to a too small reservour, cuase being 6 foot six it would mean heel and toeing would be impossible for me...my ancles only have so much rotation ;-).

Handy tip for beelding clutches which has worked really well for me, and perhaps others have not heard of it. (for alfetta platforms) Get a suitably sized peice of wood (a bar does not really work), get under the car and lever back against the fork on the slave. Then get someone to pump the pedal up, then crack nipple, lock off and repeat. This gets the pedal up very fast in my experience.

MD, interesting you say that the rears have to be bled together, could you explain why? I can't make sense of that one, but then I always end up with some air somewhere so I'm doing something wrong! Not quite true, my new boosterless system has a rock hard pedal with no air, buts thats not really factory anymore.

MD · October 21, 2010, 04:35:34 PM

Yeah hi Mike,

A long time ago I picked that advice up from a factory manual or some such document and it would up getting locked in the cranium. Now it's my set procedure. Set up the two bleeder hoses into the one bottle and open the bleeder nipples whilst the reservoir is under 10psi pressure. Good idea to flush the fluid all round every 18 months or so if not sooner(road car). Would you believe a Daihatsu manual said to replace the brake hoses every 18 months also !! How old are your hoses again ? Speaking of hoses, take care with some OE replacement hoses. I fitted a full set only to find that within 6 months, the bastards started to bubble. Talk about being pissed off !!

Now I get local Jap units made to Alfa specs. Brilliant.

Brad, bit of caution. Fitting a larger bore MC may reduce your excessive pedal travel but it may also increase the pedal pressure required substantially. This may need a revision of your pedal length fulcrum ratio so if you are going to stuff around with these things, please consult someone on the ground locally who is in this game so you wind up with some hair left and money well spent.