Be Seen On A Bike

I think running a front wheel and a 27in rear wheel 700c (larger). I've seen bikes with larger front wheels and I see no problem with that, but my fear is that if I am braking downhill with a wider rear tire, I can go on the handlebars. Is it safe?

Well ..... your logic is running in reverse. Really not a problem at all, but if it did, you would have the largest Ferris wheel in front to offset the balance of your theory of weight in the rear (offset, somewhat, the angle Hill, you are descending).

Both wheels are near the same size for all practical purposes, but you may experience problems getting your brake pads to set fairly high / low in the brake arm to contact the surface of the rim. Honestly, if ... just straighten your arms slightly and place your buttocks towards the back if you need to brake suddenly on a slope. Most road brakes are not strong enough to get you started on the bars anyway, unless you go very slow .... many can not do it at all. But shift your weight and you'll be fine.

700c and 27 "are so nearly the same size, they are close enough to work the brakes on the wheel 700c. The 700 is just alittle small.

Why you think you go to your handlebars with a wider rear wheel? It will not matter at all. The only thing to watch is that the brakes fit well with a small wheel - and they must, as mentioned in the first response.