Spongy brakes, 914-4 brakes won't firm up??! Options

[Gearren]

I've had it! I have done everything to the brakes on my car, and I can't seem to get them to stop the car with authority. Best I get is a reluctant coast to a stop. Hardly suitable for street use! I have owned the 73 2.0 since new, and have kept it in the garage since 85. Brakes have always been weak, so as I starterd on the quest to revive the car, the brakes were top on the list. I have rebuilt both front and rear calipers, turned the rotors, new pads, new M/C (17mm), bled the m/c, bled the prop valve, and bled the system at least 10 times! I still have weak/spongy brakes! I want to keep the car stock, however I am tempted to remove the prop valve to see if it is the culprit. Any ideas or suggestions would be appreciated??

[ArtechnikA]

didn't see that you adjusted the parking brake venting clearance; it's critical to good pedal feel. the value published in the Haynes manual is not correct - it's aty least twice too big.

if the venting clearance is too large it will really show up with the sstock 17mm master cylinder. many people mistake the slightly longer pedal travel and increased sensitivity for being spongy. it is possible to use the 911/914.6 19mm master cylinder if everything else has been done correctly and you still don't like the feel.

914's hydraulic system is hard to get bled properly, and the proportioner valve is a major culprit. i don't recommend removing them in an otherwise stock car with otherwise stock brakes, but i know that a lot of people do,

i have used a lot of bleeding techniques and now i've pretty much settled into pressure bleeding while at home and close to the airsource, and using the SpeedBleeder bleedscrews at the track. make sure you're using good fluid -- lots of people alternate between Ate Super Blue and Gold so they can tell at a glance when flushing is complete; i like Valvoline SynPower fluid. just make sure it's a reputable DOT 4 fluid from a fresh container and IMO - especially since you are experiencing sponginess - avoid silicone.

[smg914]

I'm not saying I recommend you remove the proportioner valve but I can tell you that I replaced the valve with the "T" connector on my old 914-4 race car and the 914-6 race car and various 914 street cars over the years with nothing but positive results.

I would start with the parking brake venting clearance and if you are still not happy, then remove the proportioner valve.

[seanery]

If the car has been sitting, the rubber lines could be past their prime. If you want to keep it original-ish, there are replacement rubber line, or you could get DOT stainless braided.

[SLITS]

(IMG:style_emoticons/default/agree.gif)

Change the rubber brake lines and see what happens.

Also, the venting clearance on the rears is critical - I use 0.004"- rock solid

[davep]

(IMG:style_emoticons/default/agree.gif)

I use new rubber lines, a 19mm master, and a tight venting clearance. I really like the old fashioned 2 person bleeding system best.
One thing that could be affecting you is wobble in the turned rotors. One way to test this is to pump up the brakes while at rest. If it feels hard the second or third time you put your foot on the brake, then the system should have been bled properly. If it feels soft, then either the lines have some bubbles or the rubber lines are flexing. If the brakes feel hard while stationary, but get really soft while rolling, then the rotors may be pushing the pads back too far, thus requiring pumping of the brakes to get the pads to work. Rear rotors are more prone to wobble due to being mounted on the hub. If the mating surfaces are not perfectly clean and smooth you will have excessive wobble, and thus excessive venting clearance. Setting the venting clearance manually will not correct the clearance generated by wobble. Note that the design of the system deliberately uses wobble to generate venting clearance; that is how the front is set up. Yes, the piston seal O-rings also contribute to this, but the amount of wobble is the maximizing factor.

[Gearren]

I neglected to say that I did set the venting clearance in accoradance to Haynes manual several times. You're saying that Haynes is 2x too large a clearance? I'll have to try reducing the clearance.

Years ago I switched the rubber brake lines to stainless steel braided. At that time 83? I also switched to a 19 mm m/c, which made the pedal harder to push, but did little to slow the car any better. This time around, I elected to go back to a stock 17mm and see if I could improve the actual stopping power of the brakes. I can live with alittle more pedal travel, but I need for the car to stop when I want it to. It seems like the pads just don't exert enough force on the discs.

I have been bleeding the system with the 2 man system, which has the 2nd man getting pretty tired. I am using Dot 3 brake fluid (Prestone), could the fluid be a contributor?

I've read some threads about losing the prop valve and installing a "T" fitting, and it seems like an
interesting idea.

[ArtechnikA]

You're saying that Haynes is 2x too large a clearance?

...I am using Dot 3 brake fluid (Prestone), could the fluid be a contributor?

I've read some threads about losing the prop valve and installing a "T" fitting, and it seems like an
interesting idea.

yes. 0,004" seems to be about right.

DOT 3 is not appropriate for disc brake systems.
DOT 4 is. there are lots of goot ones that work just fine available at the FLAPS - that's where i get my Valvoline SynPower fluid, and Castrol (might be labelled Lockheed/Girling) LMA/GT works.

the proportioner valve keeps the rear brakes from locking up first, which is A Bad Thing.
i pitched mine years ago, but it was a /6, and i uprated the front calipers, so the overall balance remained about the same. if you go that route, just be real careful about putting heavy stuff up front while leaving the back trunk empty ...

[lapuwali]

Pad material can make a significant difference. You said "new pads", but not what pads.

Stainless lines don't live forever, either. If they're 20 years old, as I think you meant, then replacing them wouldn't be a bad idea.

As mentioned, improper venting clearance will require not only longer pedal travel, but pumping the pedal should firm it up. If you press the pedal hard and hold it down, it shouldn't continue to sink even if you hold it down a long time, or you have a leak somewhere (perhaps internal). If pumping doesn't help, it's not a venting clearance problem.

[seanery]

(IMG:style_emoticons/default/agree.gif) with james.

[davep]

The early stainless lines were not DOT. They tended to be oversize lines and can balloon a little under pressure and this can contribute to a soft feel. The prop valve reduces the pressure to the rear calipers and helps to keep the car from swapping ends. Depending on many factors including driver skill, this may be a good thing or not. For the factory, at the time, it was a good thing.

As much as anything else, I don't think the size of the master makes a big difference except that when the caliper piston sizes get too large, and require a large amount of fluid to move them the required distance, in which case it may require two pumps of the master to get effective stopping power. Two pumps of a small master to active big brakes is not considered acceptable due to the time required.

Manually setting the venting clearance on the rear calipers is the starting point. Once the car starts rolling, the wobble in the rotors will set the dynamic venting clearance. If the dynamic venting clearance is too large, then too much pedal travel is required to get pad engagement. The factory manual shows how to use a dial guage to measure disc runout, and they provide specifications on the allowable value.

Your pads may be glazed, or too hard of a compound to properly bite the rotor. I think your DOT 3 fluid should be okay. I do like Castrol LMA to reduce the water absorption and corrosion inside my system. There is a lot of factors at play and it takes a methodical approach to reach a solution. Even your tires may be too hard of a compound and don't grip well enough. Power assist brakes along with ABS make modern cars stop well, but it is hard to compare to a 35 year old design. Sometimes we just expect too much. Then we can justify spending money to create a system that meets expectations. (IMG:style_emoticons/default/beer.gif)

[tat2dphreak]

I did not see this mentioned (though I did just "scan") I have found it important to bleed the prop valve by cracking the top line... make sure there is a towel over it...

this along with everthing else mentioned should help.

[mike_the_man]

I'm in about the same boat as you. I would recommend replacing the flex lines. If they haven't been replaced since 83, that could very well be your problem. I personally don't like the SS lines, but opinions vary. Next, try replacing the proportioning valve with a T, just to eliminate that as a problem. If that doesn't fix your problem, check each caliper seperately. Here is how I did that:

Either by a metric bolt that you can use as a plug, or go to your FLAPS and buy some bubble flaired brake lines. Cut them in half and compress the lines and roll them so that it seals it up. You'll need 4 of them. Now, disconnect the hard line that connects the caliper to the flex line at each corner, and use your plugs in the end of each flex line. Stomp on the brake pedal. It should be rock solid, or very close. If not, you probably have leak somewhere.

Now connect the rear passenger side caliper. Bleed it till you get a rock hard pedal. Now connect the rear drivers side caliper, bleed. repeat for each caliper. If you hit a caliper and can't seem to get a solid pedal, then there's your problem. Thats what ended up being my problem. My front drivers side caliper was sticky, and I couldn't seem to unstick it. I ended up replacing the front with BMW calipers, and they seem to work much better.

I hope this helps, or at least makes sense!

[Eric_Shea]

I have a couple good proportioning valves if you want to try a different one.

I also have one of these if you want to try it (warning Andy shot to follow): (IMG:style_emoticons/default/laugh.gif)

[Gearren]

I guess my plan of attack is to readjust the venting clearance to .004 and see if that helps/solves the problem. Then I will replace the SS lines with newer ones, and switch the fluid at that time to Dot 4 using a pressure bleeder and see what happens. If I'm still not right, I guess a replacement prop valve would be the next logical step, then a "T" fitting last. I figure 10 gallons of fluid ought to see me through this process!

For Eric_Shea, How much would you be interested in letting go of one of the prop valves and the "T" fitting?

Back in 83 I garaged my car, because I played hell trying to find parts and support in this part of the country. Things are sure looking up! Thanks for all the help! I'll keep you posted...Mike

[phantom914]

Why hasn't anyone recommended specific pads? I haven't recommended any for the 914 because I haven't tried any yet, but if the worry is just as much about stopping power as about "sponginess", I think the right pad is critical. I found this out on non 914 cars. Hawk has worked for me on another car and Porterfield seems to be recommended here sometimes (again, I have no first hand experience).

Recommendations based on first hand experience anybody?


Andrew

[ArtechnikA]

Why hasn't anyone recommended specific pads?

because the "right" pad is highly dependent upon specific application, and the intended use of the vehicle in question has not yet been mentioned.

i'm using MetalMasters on my street car; i'd recommend them for a street application. there are other pads that are okay on the street.

but this is definitely a place where counterintuitively, "race brakes" don't stop well on the street. they achieve their high performance only at high temperatures and are little better than blocks of ice for that first stop sign on cold brakes in the morning... once hot they work great - but some of them achieve their high friction levels by being extremely abrasive on disks. good disks are expensive and i don't like replacing them any more often than necessary, even if i -can- do it without a bearing change on my car. (actually, bad disks are expensive too, because you'll use up 3 bad disks in the same usage as one good one ...)

IMO - go with the softest pad that won't disintegrate in its intended use.

i thought it was interesting to hear a NASCAR crew chief say the same thing about one of their short-track cars. while the total energy dissipation is the same, a soft pad will slow you faster, which means you can stay on the gas longer, and because it will slow faster to cornering speed, it means you can get -off- the brakes sooner, giving them more time to cool.

but 30 yrs ago, endurance cars were changing brakes about every 6 hrs - 3x a race, and now they MIGHT change pads once, at half distance - and the cars are going faster and stopping shorter than they did back then. the technology has come a long way...

[smg914]

The "T" fitting can be purchased for $7.00 at most VW bug parts stores.