quiquedb

Hi dear sirs, I read you for a long time specially this forum. Sorry about my english is not good enough to talk and write here.

I am doing 964 for competition, till now the only one problem that I have is with the tact of the brake. It is very spongy. The car is a 90 unit. Last saturday we went to the Ascari circuit at Ronda, beautiful landscape, rainning day but we enjoy the day to much. The feel of the brake is terrible, is not like my 964 RS, is very spongy and suddenly the car brakes a lot. I can not understand why.

We have put 993TT front calipers and 964 RS in the back, each one with his corresponding discs, Endless pads.
We have changed the brake master cylinder for a 993 C2 one and have installed a bigger brake bottle with Castrol SRF.

I do not deal as the tact of the pedal it is so spongy and I like some orientation in the matter and helps to be able to solve this problem.

Thank you

LouZ

First, I will assume that there is no air in the entire braking system.

When you changed out all of the brake parts, master, calipers, etc., did you change the brake lines? the rubber lines to each caliper will break down over time to the point that when you apply the brakes, the lines swell (balloon) a bit, which would account for the spongy feel. New stainless steel lines to each caliper would solve this particular problem.

boxsey911

Welcome Quiquedb. Just to say that even standard 964C2 brakes are not normally spongey on the track. A complete and thorough re-bleed of the whole system with a pressure bleeder would be my first step. Unfortunately that's going to be quite expensive since you are using SRF.

Frank 993 C4S

I agree with the thorough re-bleed. What brake pads are you running?

race911

Is the master cylinder on a vacuum booster 964C2 capable of dealing with the bigger calipers? (I always ran stock brakes on my RSA racer, since it was in stock class and never had a braking issue.) I'm sure the hydraulic boost C4/RS system has a larger master cylinder, right?

Bill Verburg

correct, the electro-hyd boost can use either a 23.8 or 25.4mm m/c, the 993 vacuum boost only a 23.8.

an RS whether 964 or 993 will have a 25.4, the larger the m/c the higher and harder the brakes will be

Mc.Queen 964

Hi Quique,

I'm Alberto, Ex-Porschistacanario un Soloporsche, Nice to see you here.., I saw your proyect in SP and.. What a machine...

I really can't help you with your problem, but here you've got best people for this kind of questions.!

sergiDA

Quique good luck with your search I am sure you will be very well advised here, of course not by myself but many experts...

Why don't you post some photos of both the RS and the Cup recreation?

Sergi,

cobalt

I would have to agree with others that you must have or are drawing air into the system somehow. Do they start off spongy or gradually get spongy?

I have almost the same brakes as you describe and my pedal is rock hard. The car is driven hard with2 drivers usually running back to back 4 hour long sessions in a day and I never have a spongy pedal even after 4 or 5 events.

I had situation with a 79SC i used to track always ended up with a spongy pedal. Couldn't figure it out after replacing almost everything. I finally found a small leak in one of the blue reservoir hoses that would draw air in but never leaked fluid out. You might check all these components to see if they hold vacuum.

quiquedb

Hi my spanish friends, nice to meet you here than at our forum... You are a kindly people, except you Sergi, you are a Barsa man, then I hate you any way... 5 times... my Lord, what a day you have, my team were a silly thing... terrible... any way, I hate you

Ok... I have not air in the brake lines., I am sure, twice we re bleed it at Ascari circuit and my team is quiet proffessional, they are the Spanish winners of the Spanish GT 2009 GT championship... then they know as they can about brakes and other things, but we are lose about the feel of our brake pedal. In that way I need some help... Bill, come on, give me a chance to think about another thing with my brake system.

Sergi, I do not have photos of my CUP recreation, still waiting you to run the 500km... jejeje Esto de hablar en ingles con unos españoles lo llevo fatal... jejeje

Thanks all for your interest, I will wait more information

Dan Jacobs

Loose wheel bearings or worn hubs will give you inconsistent brakes on these cars. Are the brakes worse on one particular part of the track? Are you pumping with your left foot as you approach the brake zone?

LouZ

I still question the lines if still the original rubber.

Jimjacqmx5

I recently went through this with the guys on the forum after changing my master on my 964 C2. It took several attempts with a power bleeder and bucket loads of SRF to finally get a decent pedal back. It was an absolute nightmare and took many sessions over several weeks before we finally got it right.
The guys woking on the car are Porsche trained service engineers and were scratching their heads as much as i was. We looked at everything from the booster to the master to pedal throw adjustment to ABS unit and i the end, it was air. Absolute nightmare.

ian89C4

assuming that the ABS system is still in the car, try bleeding the ABS "bomb" reservoir in the front trunk. I had the same feel when I changed over to braided lines and in the end there was air bubbles in that.

Also try the clutch slave cylinder. It is hydraulic and uses the brake fluid reservoir - which I am sure you knew - but sometimes air gets caught up in that line as well.

In my very amateur opinion, I would say that from the description there has to be air in the system as well.

Also, what type of pads are you using?

quiquedb

Thanks everybody, I am not completely sure about if the brake system can contains air bubbles. We re-bleed the lines and all were ok, need more time to think about it.
Dan, the I felt the pedal always the same, and I bomb it with my foot and nothing to do... I want the feeling of my 964 Carrera RS!!!