Early 924 Braking Performance

[Jay H]

All,

I have a question in regards to the brakes on an early '79 924 that I have. I'm well versed in similar vintage 911's (due to currently owning one and having previously owned quite a few 911s). How should an early 924 brake as compared to a similar vintage 911? I assume that performance will be less due to the solid rotor fronts and drum rear brakes. But, should a 924 brake reasonably well if the master cylinder is new, rubber brake hoses are new, solid lines are corrosion free, rotors are in good shape (nearly zero wear), rear drums are decent and new pads and shoes are fitted? A brake fluid flush produced nearly black fluid and many bubbles in the driver's rear line. The fluid flush resulted in slightly better brakes, but not by much.

The 924 that I have has pedal feel that is average at best (long pedal travel before a good bite) and stopping distances are long. I'm am hesitant to drive the car in modern traffic (and just keep to myself on quiet backroads). Or, am I expecting too much from 1970's braking technology?

Opinions and advice are welcomed

[djnolan]

On the later 944 the rear discs are larger than most other cars around today. If you get all of the air out of the rear lines it removes the front bias and the car stops pretty well. I drive mine confidently in bumper-to-bumper rush hour traffic everyday

[Jay H]

Thanks for the quick reply and your comments are welcomed. I'm wondering now if the rear brakes on my car are even effective with all the poor fluid (fronts were MUCH clearer) and bubbles coming out. I'll re-bleed tonight...

[kdjones2000]

Your early 924 does not have self-adjusting rear drum brakes. You need to manually adjust them periodically.

That is likely most of your issue.

Once you get the rears adjusted properly, you should have fine braking performance. We used to race a '78 924 with the 4-lug suspension, and the brakes worked fine, even for 24 hour races.

[Jay H]

Thanks to both for the comments! That's renewed my faith in Porsche brakes!

My apologies for the ignorance on rear drums on these cars...

[Rasta Monsta]

If you're serious about the car, I'd spring for a 4 wheel disc conversion. Was a factory option in 79, and can be done dirt cheap.

[Jay H]

Originally Posted by Rasta Monsta

If you're serious about the car, I'd spring for a 4 wheel disc conversion. Was a factory option in 79, and can be done dirt cheap.

Thank you. Great suggestion!

I just bought the car on a whim...after owning 911's for over two decades. It was too nice (mechanically) to pass up being a car with 96k and a decent undercarriage for a Rust Belt car. I think if I can sort out the drums and get this thing to brake like a normal car (seriously, my daughter's 2014 Chevy Spark will out brake this 924 by a mile), I may think about a disc conversion.

[Jay H]

Quick Update:

I adjusted the rear brakes, but they seemed to be in pretty good adjustment already with the shoes quite close to the drum which goes with what the seller noted about the rear brakes recently being gone through. The inspection holes revealed fresh, clean looking shoes.

I bled the rears again with lots of bubbles on the driver's rear line once again. Got everything back together and fired it up. First couple pushes of the pedal were a massive improvement, then things went downhill from there. But, let the pedal alone and the feel came back quite a bit. Pump the brakes several times and the pedal feel goes away completely; nearly to the floor after 4-5 applications of the brakes in succession.

Maybe signs of a failing brake booster or vacuum issue?

Thoughts?

[petrolhead611]

A friend has a rear drum braked 924, the Euro 125 bhp version, not your 95 bhp one, and the brakes are more than sufficient for use in modern traffic, and we have plenty of that in the UK.

[djnolan]

Does it do the same thing with the engine off? If so it may be a fluid leak or a rubber brake line is possibly getting ready to go. The pedal should feel firm and the travel should not vary.

[Jay H]

Petrolhead: Thanks for the confidence building statement that I can get my car to brake better. I took the car out for the first time this morning on back roads (at an easy pace) and the brakes were better than the trip home from the seller's house. But, they still have a mushy feel and way too much travel. Side note: 110 hp is what the '79 owner's manual in my car's glove box reveals for an HP rating.

Originally Posted by djnolan

Does it do the same thing with the engine off? If so it may be a fluid leak or a rubber brake line is possibly getting ready to go. The pedal should feel firm and the travel should not vary.

With engine off, the pedal travel is very short and the brake pedal is very firm. Each successive push of the pedal repeats the same short stroke and firm feel (that typical Porsche "brick wall" that I'm used to with the 911s) with no variation. Rubber brake lines from drums and calipers to hard lines are new. All of the hard lines under the car look clean and free of any corrosion.

Any additional thoughts are very much welcomed!

Jay
79 924
84 911

[dgcantrell]

If you've got all new rubber lines on it, I'd say there is still some air trapped in it somewhere. Pressure bleeding is about the only way to assure no air in the system. It allows you to run a LOT of fluid through it. I'd invest in one of those. I've got the Motive Power Bleeder and it's great. I got several years ago for about $70. Fill it with a gallon of fluid and attach it to the master cylinder (the one I have is for Porsche/Audi's). Pump it up (like a garden sprayer) and go to each wheel and crack the bleeder and watch it till clean (no air) fresh fluid comes out. Be SURE to keep an eye on the Power Bleeder Reservoir and not let it empty out. But a gallon goes a long way as there is probably less than a pint in the entire system.

[Jay H]

Originally Posted by dgcantrell

Pressure bleeding is about the only way to assure no air in the system. It allows you to run a LOT of fluid through it. I'd invest in one of those. I've got the Motive Power Bleeder and it's great.

Thank you for the feedback!

I am using the Motive Power Bleeder and am on round two of bleeding. But, I bet you are right in that there is still some air in the system. The first round of bleeding produced more bubbles than a kid's bubble bath. The second round had less bubbles, but many still came out. Black Friday's shopping list will include another gallon of brake fluid!

[Jay H]

Very quick update...

Did the 3rd bleed with the Motive last night and drove the car today. Braking performance is steadily improving and is greatly changed (for the better) from when I bought the car a week ago and probably had only front brakes.

The car goes away for the winter now, but I'll bleed the system again a couple more times in spring to see if I can continue to try to obtain a solid pedal when the car is running.

Again, thanks for the help!

[Rasta Monsta]

Originally Posted by Jay H

First couple pushes of the pedal were a massive improvement, then things went downhill from there. But, let the pedal alone and the feel came back quite a bit. Pump the brakes several times and the pedal feel goes away completely; nearly to the floor after 4-5 applications of the brakes in succession.

Bad master.

[Jay H]

Originally Posted by Rasta Monsta

Bad master.

What is the best way in determining if the master cylinder is indeed failing? It's newer (replaced within the last 15 years / 30,000 miles) and not the original, but I suppose all bets are off given that timeframe above...

Thank you!

[Rasta Monsta]

You already did it. Bleed, adjust, test. If pedal gets worse with every step, there is a hydraulic failure.

[Jay H]

Originally Posted by Rasta Monsta

You already did it. Bleed, adjust, test. If pedal gets worse with every step, there is a hydraulic failure.

Thank you.

It is indeed improving with each round of bleeding.

[Jay H]

Quick Follow Up:

My trusted shop confirmed my thinking (and this board thinking). The brake booster has failed and there is a vacuum leak in the system. A new unit is no longer available anywhere on the planet, so a rebuilt booster is being sourced from Parts Heaven. I'm hopin' for much better brakes (and a better running car with no vacuum leak) once this unit is installed.

I'll post a final update once the car is back on the road.

[v2rocket_aka944]

Oh god, stop while you're ahead and still have both kidneys.

There has to be another option than Part$ Heaven? Did you check with Ideola?

Depending which M/C you have they are still available:
http://www.pelicanparts.com/More_Info/477612107F.htm?pn=477-612-107-F-OEM&SVSVSI=984

http://www.pelicanparts.com/More_Info/477612107G.htm?pn=477-612-107-G-OEM&SVSVSI=984