Brakes don't Bite Options

[bkrantz]

As part of my rebuild I completely replaced the brake system:
calipers and rear pressure valve rebuilt by PMB
new ATE 19mm master cylinder
new hard and flex lines
new disks and Porterfield R4-S pads

I went through many (many!) episodes of bleeding, with different methods. The pedal feels very firm from the first push, and does not pump up. Rear clearance is set at .004". I went through the PMB bedding process, and have driven around a bit.

But the braking force seems weak. When I get on the brake pedal, I do not feel any "bite". Another 914 friend drove the car and also agrees.

Any suggestions?

[Superhawk996]

Brake feel is highly subjective.

The obvious thing is that you’ve gone to the 19mm master cylinder. That involves a trade off between having less brake pedal travel but that comes at the penalty of higher brake pedal force to achieve equivalent brake line pressure vs the OEM 17mm master.

The other is to make sure you aren’t comparing expectations between modern power assisted brakes to a pure manual brake system.

The most objective question is: can you lock up the front wheels? If not, something is wrong.

[mgphoto]

19mm master cylinder, push the pedal harder, they will bite.

[barnfind9141972]

I did the same thing and also couldn’t get much brake pressure. I noticed while bleeding, there was air in the rear proportioning valve and still some in the calipers. I cracked those hard lines, let them pour out, tapped all four calipers with a rubber mallet to get more air bubbles out and re bled again and was able to get pressure. Once driving I did the aggressive braking procedure, after once or twice I finally got them to lock up and about after a week or so of regular driving did another bleed just in case. Was definitely a head banging process but I figure if the car moves it needs brakes so well worth it

[Mikey914]

The system is notorious for holding bubbles. You will have to dive it a bit bleed again, and perhaps repeat a few times.

[ChrisFoley]

As part of my rebuild I completely replaced the brake system:
calipers and rear pressure valve rebuilt by PMB
new ATE 19mm master cylinder
new hard and flex lines
new disks and Porterfield R4-S pads

I went through many (many!) episodes of bleeding, with different methods. The pedal feels very firm from the first push, and does not pump up. Rear clearance is set at .004". I went through the PMB bedding process, and have driven around a bit.

But the braking force seems weak. When I get on the brake pedal, I do not feel any "bite". Another 914 friend drove the car and also agrees.

Any suggestions?

Two suggestions:
Add a master cylinder brace to prevent floorboard flex.
Adjust the rear brake regulator to begin regulating at a higher pressure.

[Superhawk996]

Adjust the rear brake regulator to begin regulating at a higher pressure.

Bad plan

The regulator / proportioning valve doesn’t affect initial bite as brake line pressure is ramping up on moderate or even on hard brake brake applications.

The regulator only limits pressure to the rear brakes after pressure has built to the cutoff or knee point. After that point it continues to build pressure to the rear brakes but at a lesser rate than the front. By the time the regulator comes into play (on street use), weight has been transferred to the front axle and the front brakes are doing the vast majority of the work.

The regulator is there and adjusted to prevent lock up of the rear brakes and the inherent vehicle instability that comes with rear brake lock.

Messing with rear brake bias is fine for racing application but shouldn’t be taken lightly for street applications.

Here is the data on how the rear brake pressure is regulated:


48 kg/cm2 is 683 psi - that is significant brake line pressure before any reduction in rear brake pressure build rate even begins.

[JamesM]

As part of my rebuild I completely replaced the brake system:
calipers and rear pressure valve rebuilt by PMB
new ATE 19mm master cylinder
new hard and flex lines
new disks and Porterfield R4-S pads

I went through many (many!) episodes of bleeding, with different methods. The pedal feels very firm from the first push, and does not pump up. Rear clearance is set at .004". I went through the PMB bedding process, and have driven around a bit.

But the braking force seems weak. When I get on the brake pedal, I do not feel any "bite". Another 914 friend drove the car and also agrees.

Any suggestions?

Can you lock the brakes up if you stand on them?

If yes then they are working.


Pad type can change the feel of the initial "bite" however I find that the rebuilt calipers feel a little soft at first but will become more responsive over time as the new rubber in them work hardens.

I find going REALLY aggressive with the bedding procedure helps with this as well. You really want to cook those things.

[Superhawk996]

Add a master cylinder brace to prevent floorboard flex.

(IMG:style_emoticons/default/agree.gif)

And Tangerine has a nice kit to do so!

[Superhawk996]

Can you lock the brakes up if you stand on them?

(IMG:style_emoticons/default/agree.gif)

This question needs to be answered. If the front brakes can’t be locked there is something wrong.

[jim_hoyland]

What MPH would the front lock-up test best be done ?

[Superhawk996]

What MPH would the front lock-up test best be done ?

30- 40 mph is sufficient. Dry pavement. Safe area to do so.

You want the vehicle to have enough momentum so that it will take quite a bit of line pressure to lock the brakes but not end up stopping the vehicle completely.

A light squeal or heavy scuffing from the tire is all it takes to confirm No need to keep it locked for any time/distance that would cause a tire to flat spot.

[ChrisFoley]

Adjust the rear brake regulator to begin regulating at a higher pressure.

Bad plan

I've done it enough times to know that it works really well.

[Superhawk996]

Adjust the rear brake regulator to begin regulating at a higher pressure.

Bad plan

I've done it enough times to know that it works really well.

I’ve been a brake design engineer long enough to know that it is a really bad plan. I’ve also done enough brake testing (with fully instrumented vehicles) and regulatory signoff to back up the theoretical side of design engineering.

Rear brake line pressure needs to account for varying road and tire friction, vehicle load variance like a fully loaded front trunk (full fuel + luggage) with no load in the rear, and not locking up while cornering with lateral load, etc.

I’ve posted the data. Below that threshold pressure, the regulator is not doing anything - it passes pressure straight through. There is nothing there that will affect initial bite that is occurring well below 683 psi of line pressure.

[emerygt350]

I just went and tried Locking them up tonight (haven't done it before, and was never impressed with my brakes), but darn if they didn't lock up nice when I gave it the old what for. Just the fronts of course.

[bkrantz]

Thanks for all the replies.

Yes, I can lock up the fronts. It takes a good shove, and maybe the left locks before the right.

I hear you about bleeding and stubborn air. Like I said, I went through several episodes with different techniques. And I will do more, with all the chanting and banging on components I can think of.

I went through the bedding process again today, and maybe the brakes feel a bit better.

And I realize that 1960s engineering, and small, single piston calipers can only do so much. But I still think I should feel more bite and urgency when I get on the brakes hard.

[Van B]

I love the motive pressure bleeder. I don’t know how I lived without it all these years

[bbrock]

I have essentially the same setup as you and get really good bite. After I bedded in the Porterfield pads following Eric's procedure, I could smell the brakes strongly inside the cabin and see smoke rolling from under the fenders. In other words, I really gave them hell but they never faded. After that, I could feel a distinct difference in bite.

[Superhawk996]

But I still think I should feel more bite and urgency when I get on the brakes hard.

OK - so we know there isn’t a major defect or massive air in the system if you can lock the brakes.

The desire for more “bite” is a fair feeling to have. However, what we do know for sure is that you can generate lock but you don’t like the effort (pedal force) required to do it.

The problem will be trying to tune a system via the internet.

My first recommendation would actually be to install the Tangerine reinforcement to make sure as much force gets to the master cylinder as possible.

My second would be to recognize that you went to the 19mm master cylinder which is working counter to what you want to achieve - which is having less pedal force get you to an equivalent line pressure. The problem here is 1) finding a 17mm master 2) reopening the hydraulics to install it and having to redo the bleed.

Finally, I suspect you are indeed expecting a manual brake system to perform like a power assist brake system we all have become so used to. A manual brake system is largely linear. More pedal pressure is needed to generate more brake output pressure.

Vacuum assist and modern electronic brake boost systems can provide non-linear output that cannot be matched by a manual brake systems. I’ll find a graph to show what I’m talking about.

Note how a vacuum booster initially has lots of output at the front of the curve (green and magenta regions) but then tapers off (blue) - I suspect this lis the sort of “bite” you’re looking for.



Look at the yellow region in the bottom left- there is almost a vertical region in the performance curve. Very little increase in pedal force is getting a lot of output. This “jump in” can’t be duplicated by manual brakes.

[iankarr]

It takes several bleeds to get air out of a totally fresh system. Put speed bleeders in and stomp the pedal in conjunction with a motive pressure bleeder at 10psi

Even when all the air is out, Eric at PMB says it will take several hundred miles of driving for the piston seals to wear-in. I had the same thoughts about my pedal as you. Eric was right.

You didn’t mention if you bled the MC first. Definitely should.